Source: http://www.cisg.law.pace.edu/cisg/principles/uni18.html
Timestamp: 2019-04-21 11:15:04+00:00

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1. A statement made by or other conduct of the offeree indicating assent to an offer is an acceptance. Silence or inactivity does not in itself amount to acceptance.
2. An acceptance of an offer becomes effective at the moment the indication of assent reaches the offeror. An acceptance is not effective if the indication of assent does not reach the offeror. An acceptance is not effective if the indication of assent does not reach the offeror within the time he has fixed or, if no time is fixed, within a reasonable time, due account being taken of the circumstances of the transaction, including the rapidity of the means of communication employed by the offeror. An oral offer must be accepted immediately unless the circumstances indicate otherwise.
3. However, if, by virtue of the offer or as a result of practices which the parties have established between themselves or of usage, the offeree may indicate assent by performing an act, such as one relating to the dispatch of the goods or payment of the price, without notice to the offeror, the acceptance is effective at the moment the act is performed, provided that the act is performed within the period of time laid down in the preceding paragraph.
Under the Convention and also under the UNIDROIT Principles, the contract is concluded when an offer is accepted.
"A statement made by or other conduct of the offeree indicating assent to an offer is an acceptance. Silence or inactivity does not in itself amount to acceptance."
In general, it is accepted that under both instruments the contract is perfected when the offer and the acceptance coincide. The acceptance may be given in several ways, and it does not necessarily have to consist of an expressed act of declaration of will. Both the Convention and the Principles establish that the acceptance can consist of a declaration or an act of the offeree that indicates assent.
The acceptance of an offer can be express or tacit. The first is expressly presented as the assent of the offeree. The second, when it is an act that constitutes acceptance, is expressed by means of acts that denote such an assent, i.e., acts of contractual execution. This form of acceptance has been called "acceptance by conduct or an act of dominion".
In some cases, and especially in business transactions, it is not unusual to conclude a contract without the acceptance having been manifested in an express way, but rather tacitly by conduct. In such cases it might be difficult to precisely determine the moment and place of acceptance. However, based on other factors or elements present in previous orders or dealings between the parties, the legal effect of acceptance can be deduced from the surrounding circumstances in order to arrive at the desired effect that is the formation of the contract.
Some legal systems are not clear as to the validity of the acceptance by silence, and neither is the doctrine unanimously accepted or recognized as a likely or effective way of acceptance. As a general rule, silence is not effective to denote the acceptance of the offer. To be effective as an acceptance, silence must be accompanied by acts that allow the inference or deduction of the offeree's acceptance of the offered contract, adding as requirement that the offeree has knowledge of the offer in the terms provided by the applicable in the law. It is also necessary to mention that tacit acquiescence must be manifested in unequivocal facts of contractual execution; it cannot consist of simple manifestations of the offeree's general intention in the sense of having received the offer or merely agreeing to it. As with a proposal which constitutes an offer, the acceptance must be characterized by the presence of the intention of the relevant party to bind himself to a contractual obligation when manifesting such assent to an offer.
The above remarks point to a difference between silence, on the one hand, and acceptance by conduct, on the other hand: the mere silence or inactivity of the parties is not sufficient to conclude a contract, but when it is accompanied by what are denominated unequivocal acts of contractual execution, all those acts that entail execution of the contractual benefits or preparation of the same, it is effective.
"A statement made by or other conduct of the offeree indicating assent to an offer is an acceptance" [emphasis added].
"Silence or inactivity does not in itself amount to acceptance" [emphasis added].
It is thus clear that both under the Convention and the UNIDROIT Principles, mere silence or inaction, i.e., not accompanied by acts of contractual execution, does not have the legal effect of concluding a contract. In the first part of the counterpart provisions, it is clearly indicated that assent can be shown either by means of an express declaration of assent or by means of other conduct of the offeree indicating such, i.e., acts that can be understood as unequivocal acts of execution of the contract. In the second part, however, both provisions also instruct that mere silence or inaction does not have any legal effect to conclude a contract.
Under the Convention, acceptance by conduct has a contractual legal effect provided it is accompanied by acts that indicate assent. The text of CISG Art. 18(3) provides by way of example two instances that can be assumed as effective acts of contractual execution -- one relating to the dispatch of the goods by the seller and one relating to the payment of the price by the buyer. That provision may, however, fit many other instances and that can be appreciated further in other concrete cases. DÍEZ PICAZO mentions, for example, other such acts, such as the parties' acts towards the preparation of the dispatch of the goods or for the payment of the price, e.g., the opening of a documentary letter of credit.
CISG Art. 18(3) clearly permits the possibility of acceptance in a tacit way: an acceptance is effective if "by virtue of the offer or as a result of practices which the parties have established between themselves or of usage,[] the offeree [indicates] assent by performing an act, such as one relating to the dispatch of the goods or payment of the price, without notice to the offeror". In other words, the Convention recognizes cases in which the presence of execution acts will point out the acceptance of the offer.
The available doctrine that is relevant to those provisions of the Convention supports equally the proposition that mere silence does not constitute an acceptance. However, as pointed out above, there exist some factors in combination with which silence may constitute an effective acceptance as understood between the parties. Such factors include: legal dispositions (e.g., as set out in CISG Arts. 19(2) and 21); usages and practices, and the existence of a duty upon the parties to communicate (reply/answer to communications). We would add to such list of factors/circumstances certain cases in which the parties have agreed to a certain mode of acceptance, either in previous dealings between themselves or in the particular contract in question -- keeping in mind that CISG Art. 6 permits the parties to "derogate from or vary the effect of any of [the Convention's] provisions", thus entrenching in the Convention the principle of party autonomy and its ultimate power over (almost) any of the Convention's provisions.
In a similar way to the Convention, the UNIDROIT Principles have recognized the value of acceptance by conduct, in almost identically worded counterpart provisions, see Arts. 2.6(1) and (3) of the Principles.
Art. 2.6(3) of the Principles is almost identical to its counterpart Art. 18(3) of the Convention, only the latter includes additional wording (a) to illustrate the type of acts contemplated in both counterpart provisions ("such as one relating to the dispatch of the goods or payment of the price") and (b) to clarify the timing of such an acceptance, by directly linking that issue to the provisions contained in CISG Art. 18(2).
The latter additional content of CISG Art. 18(3), compared to Art. 2.6(3) of the Principles, does not, however, weaken or undermine the thematic and substantive similarities between the counterpart instruments and their respective provisions, because Art. 2.7 of the Principles ("Time of Acceptance") is substantively identical to CISG Art. 18(2).
Furthermore, the official commentary on UNIDROIT Principles Art. 2.6(3) expressly highlights the similarity in policy and content of the counterpart provisions: "This article corresponds to para. (1), (2) first part and (3) of Art.18 CISG."
Thus, it can be concluded that, as in the Convention, under the Principles silence alone cannot constitute acceptance. It should be made clear when the offeror unilaterally determines, as he may, the mode of acceptance, the offeree cannot argue than an effective acceptance has taken place through the latter's silence. The official commentary on Art. 2.6 of the Principles clearly supports this interpretation.
Furthermore, in accordance with the opinion expressed by PERALES, the present author concurs that for silence to constitute an acceptance, such result could only be derived from (a) an express agreement of the parties (Art. 1.1 of the Principles), (b) usages or practices established between the parties (Art. 1.8 of the Principles), or (c) when other applicable law/principles recognize an acceptance in that form (Arts. 2.9 and 2.22 of the Principles).
An acceptance can be communicated by different means, unless the offeror has demanded a specific formality. In cases where the offeror has used a specific medium to communicate his offer but did not request obligatory observance of the same means for the communication of the offeree's acceptance, nothing prevents the offeree from communicating an acceptance by different means.
An express acceptance can be made orally. Oral conclusion of a sales contract can be proved by a letter of confirmation sent afterwards by one party to the other.
Under the Convention's regime, applying the principle of freedom from formalities in ways contemplated in CISG Art. 11, a contract can be proved by many different means suitable to the offeree to communicate or prove his assent to the offer. Under the Convention, an acceptance can also be given in writing, and for such mode this expression ("writing") includes telegram and the telex, in accordance with CISG Art. 13.
"Nothing in these Principles requires a contract to be concluded in or evidenced by writing. It may be proved by any means, including witnesses."
- " 'writing' means any mode of communication that preserves a record of the information contained therein and is capable of being reproduced in tangible form."
Art. 1.10 of the Principles may be used to aid the interpretation of CISG Art. 13 in a broad manner to include modern means of communication, so that the reference to "telegram" and "telex" in the test of the Convention is not understood in a restrictive way that might exclude other modern media by which a party may dispatch a communication (e.g., messages of electronic data, and in general electronic communications).
As with an offer, it is equally important to determine the exact moment when the acceptance becomes effective to conclude the contract. That moment will depend on the provisions/rules of the applicable legal system, which might conceivably endorse a system/theory based (a) on the offeree's dispatch of his assent or (b) on the receipt by the offeror of the offeree's assent or (c) the mere communication of the offeree's assent to the offeror.
The Convention regulates acceptance distinguishing several suppositions, in the following way: pursuant to CISG Art. 18(2), first sentence, acceptance of the offer becomes effective "at the moment when the indication of the offeree's assent reaches the offeror" [emphasis added]. The theory of receipt is, thus, adopted under the Convention's regime for contract formation.
CISG Art. 18(2), in sentence two, further provides that an acceptance "is not effective if the indication of assent does not reach the offeror within the time he has fixed or, if no time is fixed, within a reasonable time, due account being taken of the circumstances of the transaction, including the rapidity of the means of communication employed by the offeror" [emphasis added].
As for offers made orally by the offeror, Art. 18(2), sentence three, provides that such offers "must be accepted immediately unless the circumstances indicate otherwise" [emphasis added].
As for the exact moment when a tacit acceptance produces the desired legal effect (i.e., to conclude a contract), it should be kept in mind that CISG Art. 18(3) is partly based on the principle that the offeror must have knowledge of the offeree's acceptance by conduct, except - and it is the exception introduced by this article -- in cases where the acceptance becomes effective either pursuant to practices established between the parties or is based on usages under which the offeree may manifest his acceptance without communicating it to the offeror. In the latter cases, it is assumed that the offeror has knowledge of the acceptance at the moment of execution of the offerees' act which configures the requisite -- and, in all respects, effective to conclude the contract -- acceptance by conduct. That is to say, the Convention permits, by virtue of established practices or usages, the possibility that the offeree indicate his assent executing an act "... such as one relating to the dispatch of the goods or payment of the price, without notice to the offeror, the acceptance is effective at the moment the act is performed, provided that the act is performed within the period of time laid down in the preceding paragraph" [CISG Art. 18(3), emphasis added]. In this case it is not necessary that the offeror receive a notification of such an executed act.
In cases, however, where the above suppositions do not apply, the offeree must communicate to the offeror the execution of the requisite act to conclude the contract, otherwise the offeror can revoke the offer pursuant to the provision in CISG Art. 16(1).
For an offer to become effective the UNIDROIT Principles, like the Convention, adopt the system of receipt, pursuant to Art. 2.6(2): "An acceptance of an offer becomes effective when the indication of assent reaches the offeror" [emphasis added]. The drafters of the Principles considered it more sensible to place on the offeree the risk of the transmission, because the offeree is the one who chooses the means of communicating his assent, and it is the offeror who is more able or better equipped to adopt the necessary measures in ensuring that the acceptance arrives at its destination.
Art. 2.6(3) of the Principles, however, like the Convention, also permits the possibility that by virtue of the offer or as a result of the practices established between the parties or pursuant to applicable usages, the offeree manifest his assent by executing an act without communicating it to the offeror. In such cases, the act of acceptance produces the desired legal effect (i.e., conclusion of the contract) at the moment when the requisite act is executed, without any necessity to make that known to the offeror; that is to say, by-passing the method of the declaration of assent by the offeree.
Nevertheless the above-mentioned would also fit the system/theory of information, in cases where a third party informs to the offeror of the acts that constitute acceptance of the proposal.
CISG Art. 24 provides that the communication of offer and acceptance, i.e., any indication of the intention of the parties in the context of contract formation under the Convention, reaches the addressee "when it is delivered to him, not when it is dispatched."
The UNIDROIT Principles, in Art. 1.9(2), also adopt the receipt principle to validate the effect of a notice or other communication when "it reaches the person to whom it is given." The Official Commentary on UP Article 1.9 explains that a "notice reaches a person when given to that person orally or delivered at that person's place of business or mailing address."
Thus, it is concluded that Art. 1.9 of the UNIDROIT Principles and Art. 24 of the Convention - the counterpart provisions that define the point of time when a communication reaches the addressee -- adopt the same receipt principle, make the same distinction between oral and other communications, and provide similar definitions of the relevant concepts.
Article 18(2) of the Convention provides that "[a]n acceptance of an offer becomes effective at the moment the indication of assent reaches the offeror. An acceptance is not effective if the indication of assent does not reach the offeror. An acceptance is not effective if the indication of assent does not reach the offeror within the time he has fixed [...]."
Where the offeror has fixed a time for acceptance, both the CISG (Art. 18(2)) and the UNIDROIT Principles (Art. 2.7) require that the offer is accepted within the set time. Otherwise, the purported acceptance will not produce the legal effect of contract conclusion, subject to exceptions contemplated in CISG Art. 21 and its counterpart Art. 2.9 of the UNIDROIT Principles.
Where the offeror has not fixed a time for acceptance, both the Convention and the Principles provide that the acceptance must arrive within a reasonable time, taking into account the circumstances of the transaction, "including the rapidity of the means of communication employed by the offeror."
Other circumstances that ought to be kept in mind to determine the reasonable term would be the object of the contract, the complexity of the transaction, geographical circumstances, etc.
In the case of oral offers, the counterpart provisions of the Convention (CISG Art. 18(2)) and the Principles (Art. 2.7) require that the same are accepted immediately.
It is also obvious that both instruments adopt the same differentiating approach to determine the duration of the offer, depending on whether the offer is in writing or oral.
Both the Convention and the UNIDROIT Principles adopt the same policy -- which is expressed in similar wording in the counterpart provisions of the two instruments -- to deal with the issue of the time and manner for indicating assent to an offer.
The counterpart provisions provide that an offer can be accepted expressly or tacitly.
Both instruments deny that silence in itself is capable of producing the legal effect of an acceptance to an offer.
An acceptance becomes effective when it reaches the offeror. Equally, an acceptance becomes effective when by virtue of usages or practices established practices between the parties, the offeree performs a relative act of execution of the contract.
If the offeror has fixed a time for acceptance of the offer, both the Convention and the Principles provide that the acceptance must take place within that time -- and where there is not time fixed for acceptance, within a reasonable time.
Both instruments also expressly provide that oral offers must be accepted immediately.
Furthermore the official commentaries on UNIDROIT Principles Arts. 2.6 and 2.7 state that these provisions correspond to CISG Art. 18.
Based on the preceding comparative analysis of the two instruments, it is submitted that the counterpart provisions and the contexts in which they are set are substantively identical. As such, the provisions of the Principles and the corresponding official comments on these provisions may arguably be used to interpret or supplement the counterpart provisions of the CISG.
* Lawyer (J.D.), Javeriana University (Bogotá D.C. - Colombia). Specialist in Commercial Law Javeriana University. Professor of Contracts and Commercial Law at La Sabana University (Bogotá D.C. - Colombia). He has been Lecturer in Colombian and other international universities. Author of publications on International Commercial Law in Argentina, Spain, Colombia, Peru, Mexico, and the United States.
1. CISG Art. 23 provides: "A contract is concluded at the moment when an acceptance of an offer becomes effective in accordance with the provisions of this Convention."
UNIDROIT Principles Art. 2.1 provides: "A contract may be concluded either by the acceptance of the offer or by conduct of the parties that is sufficient to show agreement" [emphasis added].
For a comparative analysis of CISG Art. 18 and the counterpart provisions of another Restatement of Contract law, the Principles of European Contract Law 1998, see CARRARA, C. and KUCKENBURG, J.: "Remarks on the manner in which the Principles of European Contract Law may be used to interpret or supplement Article 18 of the CISG", available online at <http://cisgw3.law.pace.edu/cisg/text/peclcomp18.html#er>.
2. See SONO, K.: "Formation of International Contracts under the Vienna Convention: A Shift above the Comparative Law", in SARCEVIC P. and VOLKEN P. (eds.): International Sale of Goods: Dubrovnik Lectures, Oceana (1986) 111-131; also available at <http://cisgw3.law.pace.edu/cisg/biblio/sono2.html>.
"The statement of acceptance does not expressly have to declare acceptance of the offer; it is necessary that assent to the offer be expressed by the offeree. The statement or conduct is interpreted pursuant to [CISG] Article 8. The statement must express assent to the offer. The mere acknowledgment of receipt of the offer is thus not sufficient, neither is an expression of interest in it."
4. CALAMARI, J.; PERILLO, J.: The Law of Contract, (1998), Hornbook Series, West Group, St. Paul Minn., p. 85.
5. See CISG Art. 9 [Usages and practices applicable to the contract].
6. SCHLECHTRIEM, P.: "Acceptance of an Offer (Articles 18-22)", in Uniform Sales Law -- The UN-Convention on Contracts for the International Sale of Goods, Manz, Vienna: 1986, also available online at <http://cisgw3.law.pace.edu/cisg/biblio/schlechtriem.html#a29>.
7. See Germany 13 January 1993 Oberlandesgericht [Appellate Court], case presentation available at <http://cisgw3.law.pace.edu/cases/930113g1.html> (the court held that a contract had been validly concluded between the parties, noting that the buyer's taking delivery of the goods constituted conduct indicating assent to the offer and amounted therefore to an implied acceptance of the standard terms contained in the letter of confirmation sent by the seller (Art. 18(1) CISG)).
- France 10 September 2003 Cour d'appel [Appellate Court] Paris (Société H. H... GmbH & Co. v. SARL MG...), case presentation including English translation available at <http://cisgw3.law.pace.edu/cases/030910f1.html> (the court held that even though acceptance may result from the behavior of the offeree, silence or inactivity does not in itself amount to acceptance).
- Belgium 2 December 1998 Rechtbank van Koophandel [District Court] Hasselt, case presentation available at <http://cisgw3.law.pace.edu/cases/981202b1.html> (the court remarked that CISG Art. 18 excludes acceptance of contract terms by mere silence).
10. See Official Comments on Art. 2.6 of the UNIDROIT Principles, available online at <http://cisgw3.law.pace.edu/cisg/principles/uni18.html#official>.
Comment 1. "Indication of assent to an offer. For there to be an acceptance the offeree must in one way or another indicate 'assent' to the offer. The mere acknowledgement of receipt of the offer, or an expression of interest in it, is not sufficient. Furthermore, the assent must be unconditional, i.e. it cannot be made dependent on some further step to be taken by either the offeror [...] or the offeree [...]. Finally, the purported acceptance must contain no variation of the terms of the offer or at least none which materially alters them (see Art. 2.11)."
Comment 2. "Acceptance by conduct. Provided that the offer does not impose any particular mode of acceptance, the indication of assent may either be made by an express statement or be inferred from the conduct of the offeree. Para. (1) of this article does not specify the form such conduct should assume: most often it will consist in acts of performance, such as the payment of an advance on the price, the shipment of goods or the beginning of work at the site, etc."
11. See Official Comments on Art. 2.6 of the UNIDROIT Principles, op.cit.
Comment 3. "Silence or inactivity. By stating that '[s]ilence or inactivity does not in itself amount to acceptance', para. (1) makes it clear that as a rule mere silence or inactivity on the part of the offeree does not allow the inference that the offeree assents to the offer. The situation is different if the parties themselves agree that silence shall amount to acceptance, or if there exists a course of dealing or usage to that effect. In no event, however, is it sufficient for the offeror to state unilaterally in its offer that the offer will be deemed to have been accepted in the absence of any reply from the offeree. Since it is the offeror who takes the initiative by proposing the conclusion of the contract, the offeree is free not only to accept or not to accept the offer, but also simply to ignore it."
12. See DIEZ PICAZO, L.: La Compraventa Internacional de Mercaderías. Comentario de la Convención de Viena, Civitas, Madrid, 1998, p. 182.
- United States 10 May 2002 Federal District Court [New York] (Geneva Pharmaceuticals Tech. Corp. v. Barr Labs. Inc.), case presentation available at <http://cisgw3.law.pace.edu/cases/020510u1.html> (the seller, a Canadian manufacturer of a chemical ingredient (clathrate) for use in the production of an anticoagulant medication (warfarin sodium), in 1994 supplied the buyer, a U.S. company, with samples of the ingredient and confirmed that it would support the buyer's application for approval by the Food and Drug Administration (FDA) as the supplier of the ingredient for the manufacture of the drug. In 1995, the seller issued a letter to the FDA confirming it would serve as a supplier of clathrate to the buyer. The court held that pursuant to CISG Art. 18(3) the provision of the reference letter to the FDA could qualify as an act indicating assent to a contract. Whether the seller's acts actually indicated assent to a contract would be analyzed at trial on the basis of industry custom).
- France 27 January 1998 Cour de Cassation [Supreme Court] (Hughes v. Société Technocontact), case presentation including English translation available at <http://cisgw3.law.pace.edu/cases/980127f1.html>.
14. See United States 14 April 1992 Federal District Court [New York] (Filanto v. Chilewich), case presentation available at <http://cisgw3.law.pace.edu/cases/920414u1.html>.
"General principles (duty to communicate)/Good faith. Winship puts a key ruling of the court as follows: '[T]he opinion may . . . be read as saying that parties in a long-term relationship owe to each other a duty to communicate, a duty which ultimately may be derived from a duty to act in good faith' (Peter Winship, "The UN Sales Convention and the Emerging Case law", in Emptio-Venditio Internationales, Neumayer ed. (Basel 1997) 228)."
"Silence could express acceptance if usages and practices that exist between the parties (Article 9) called for expressly rejecting an offer. In the case of longstanding business relations, silence for reason of good faith (c. Article 7) may mean acceptance [...]. Through an inquiry or an invitation to submit an offer it may be communicated that one's own silence should be interpreted as acceptance [...]. The parties may also agree that for future contracts silence would amount to acceptance, e.g. in the case of continuous orders [...]. It is not clear, however, at which moment the contract is concluded in the event of agreed silence [...] probably not when the offer is received but rather after a reasonable time" [references omitted].
15. FOLSOM, R.; GORDON, M.W.; SANOGLE, J.JR.: International Business Transactions (Second edition), West Group, St. Paul, Minn. United States, (2001), p. 32.
See also KRITZER, and ENDERLEIN & MASKOW, supra note 14.
16. ADAME GODDARD, J.: El contrato de compraventa internacional, Mc Graw Hill, Mexico, (1994), p. 108.
17. CISG Art. 6 reads: "The parties may exclude the application of this Convnetion or, subject to article 12, derogate from or vary the effect of any of its provisions."
"With respect to the time within which an offer must be accepted, this article, which corresponds to the second part of para. (2) of Art. 18 CISG, distinguishes between oral and written offers. Oral offers must be accepted immediately unless the circumstances indicate otherwise. As to written offers, all depends upon whether or not the offer indicated a fixed time for acceptance: if it did, the offer must be accepted within that time, while in all other cases the indication of assent must reach the offeror 'within a reasonable time having regard to the circumstances, including the rapidity of the means of communication employed by the offeror'.
"It is important to note that the rules laid down in this article also apply to situations where, in accordance with Art. 2.6(3) [see above], the offeree may indicate assent by performing an act without notice to the offeror: in these cases it is the act of performance which has to be accomplished within the respective periods of time.
"For the determination of the precise starting point of the period of time fixed by the offeror, and the calculation of holidays occurring during that period of time, see Art. 2.8; as to cases of late acceptance and of delay in transmission, see Art. 2.9."
19. See Official Comments on Art. 2.6 of the Principles, op. cit., Comment 4.
20. PERALES VISCASILLAS, M.: Comentario a los principios de UNIDROIT para los contratos del comercio internacional. p. 118.
21. Germany 22 February 1994 Oberlandesgericht [Appellate Court] Köln, case presentation including English translation available at <http://cisgw3.law.pace.edu/cases/940222g1.html> (the court held that according to CISG Art. 18(1) silence does not in itself amount to acceptance; however silence may amount to acceptance when it is linked to other circumstances -- the court also noted that importance of commercial letters of confirmation as evidence of the formation of contract).
22. 2 CISG art. 11: "A contract of sale need not be concluded in or evidenced by writing and is not subject to any other requirement as to form. It may be proved by any means, including witnesses."
23. For a comparative analysis of CISG Art. 13 and UNIDROIT Principles Art.1.10, see CHARTERS, A.: Editorial remarks, available online at <http://cisgw3.law.pace.edu/cisg/principles/uni13.html#er>.
"An acceptance becomes effective when an electronic indication of assent has entered the offeror's server, provided that the offeror has consented, expressly or impliedly, to receiving electronic communications of that type, in that format, and to that address.
The term 'oral' includes electronically transmitted sound in real time and electronic communications in real time. An offer that is transmitted electronically in real time communication must be accepted immediately unless the circumstances indicate otherwise provided that the addressee consented expressly or impliedly to receiving communications of that type, in that format, and to that address."
25. See Official Comments on Art. 2.6(3) of the UNIDROIT Principles, op. cit.
Comment 4. "When acceptance becomes effective: According to para. (2) an acceptance becomes effective at the moment the indication of assent reaches the offeror (see Art. 1.9(2)). For the definition of 'reaches' see Art. 1.9(3). The reason for the adoption of the "receipt" principle in preference to the 'dispatch' principle is that the risk of transmission is better placed on the offeree than on the offeror, since it is the former who chooses the means of communication, who knows whether the chosen means of communication is subject to special risks or delay, and who is consequently best able to take measures to ensure that the acceptance reaches its destination.
"As a rule, an acceptance by means of mere conduct likewise becomes effective only when notice thereof reaches the offeror. It should be noted, however, that special notice to this effect by the offeree will be necessary only in cases where the conduct will not of itself give notice of acceptance to the offeror within a reasonable period of time.[...].
"An exception to the general rule of para. (2) is to be found in the cases envisaged in para. (3), i.e. where "by virtue of the offer or as a result of practices which the parties have established between themselves or of usage, the offeree may indicate assent by performing an act without notice to the offeror". In such cases the acceptance is effective at the moment the act is performed, irrespective of whether or not the offeror is promptly informed thereof "[illustrations provided therein are omitted].
26. See: VÁSQUEZ LÉPINETTE, T.: Compraventa Internacional de Mercaderías, una visión jurisprudencial. Aranzadi editorial, Elcano Navarra, 2000, pp. 131.
27. See Official Comments on Art. 2.6(3) of the UNIDROIT Principles, op. cit. Comment 4.
"[...] In all other cases, e.g. where the conduct consists in the payment of the price, or the shipment of the goods by air or by some other rapid mode of transportation, the same effect may well be achieved simply by the bank or the carrier informing the offeror of the funds transfer or of the consignment of the goods."
28. See the Text of the Secretariat Commentary on article 22 of the 1978 Draft [draft counterpart of CISG article 20], Comment 1, available online at <http://cisgw3.law.pace.edu/cisg/text/secomm/secomm-24.html>.
"With respect to all kinds of notices the Principles adopt the so-called 'receipt' principle, i.e. they are not effective unless and until they reach the person to whom they are given. For some communications this is expressly stated in the provisions dealing with them: see Arts. 2.3(1), 2.3(2), 2.5, 2.6(2), 2.8(1) and 2.10. The purpose of para. (2) of the present article is to indicate that the same will also be true in the absence of an express statement to this effect: see Arts. 2.9, 2.11, 3.13, 3.14, 6.1.16, 6.2.3, 7.1.5, 7.1.7, 7.2.1, 7.2.2, 7.3.2 and 7.3.4."
30. For a comparison between CISG Art. 24 and the counterpart provisions of the UNIDROIT Principles Art. 1.9, see FELEMEGAS, J.: Editorial remarks, available online at <http://cisgw3.law.pace.edu/cisg/biblio/felemegas11.html>.
31. CISG Art. 20 deals with the interpretation of the offeror's time-limits for acceptance of an offer to conclude a contract, and it provides a mechanism for calculating when that period begins to run in cases where the commencement of the period of time during which an offer can be accepted by the offeree has not been expressly fixed by the offeror. Art. 2.8 of the Principles is a similar provision dealing with the calculation of the time for acceptance of an offer. For a comparison between CISG Art. 20 and the counterpart provisions of Art. 2.8 of the Principles, see FELEMEGAS, J.: Editorial remarks, available online at <http://cisgw3.law.pace.edu/cisg/biblio/felemegas9.html>.
32. See ICC Arbitration Case No. 7844 of 1994 , case presentation available at <http://cisgw3.law.pace.edu/cases/947844i1.html> (the arbitral tribunal held that pursuant to CISG Art. 18(2) an offer cannot be accepted after the time for acceptance has expired, unless the offeror orally informs the offeree without delay that it considers the late acceptance as effective (CISG Art. 21(2)).
33. For a comparison between CISG Art. 21 of the CISG and the counterpart provisions of Art. 2.9 of the UNIDROIT Principles, see FELEMEGAS, J.: Editorial remarks, available online at <http://cisgw3.law.pace.edu/cisg/biblio/felemegas12.html>.
34. CISG Art. 18(2); UNIDROIT Principles Art. 2.7.
35. "What is reasonable always depends on the circumstances of each case". ENDERLEIN & MASKOW, op. cit., at <http://www.cisg.law.pace.edu/cisg/biblio/enderlein.html#art18-1a>.
For there to be an acceptance the offeree must in one way or another indicate "assent" to the offer. The mere acknowledgement of receipt of the offer, or an expression of interest in it, is not sufficient. Furthermore, the assent must be unconditional, i.e. it cannot be made dependent on some further step to be taken by either the offeror (e.g."Our acceptance is subject to your final approval") or the offeree (e.g."We hereby accept the terms of the contract as set forth in your Memorandum and undertake to submit the contract to our Board for approval within the next two weeks"). Finally, the purported acceptance must contain no variation of the terms of the offer or at least none which materially alters them (see Art. 2.11).
Provided that the offer does not impose any particular mode of acceptance, the indication of assent may either be made by an express statement or be inferred from the conduct of the offeree. Para. (1) of this article does not specify the form such conduct should assume: most often it will consist in acts of performance, such as the payment of an advance on the price, the shipment of goods or the beginning of work at the site, etc.
By stating that "[s]ilence or inactivity does not in itself amount to acceptance", para. (1) makes it clear that as a rule mere silence or inactivity on the part of the offeree does not allow the inference that the offeree assents to the offer. The situation is different if the parties themselves agree that silence shall amount to acceptance, or if there exists a course of dealing or usage to that effect. In no event, however, is it sufficient for the offeror to state unilaterally in its offer that the offer will be deemed to have been accepted in the absence of any reply from the offeree. Since it is the offeror who takes the initiative by proposing the conclusion of the contract, the offeree is free not only to accept or not to accept the offer, but also simply to ignore it.
1. A requests B to set out the conditions for the renewal of a contract for the supply of wine, due to expire on 31 December. In its offer B includes a provision stating that "if we have not heard from you at the latest by the end of November, we will assume that you have agreed to renew the contract on the conditions as indicated above". A finds the proposed conditions totally unacceptable and does not even reply. The former contract expires on the fixed date without a new contract having been agreed between the parties.
2. Under a long-term agreement for the supply of wine B regularly met A's orders without expressly confirming its acceptance. On 15 November A orders a large stock for New Year. B does not reply, nor does it deliver at the requested time. B is in breach since, in accordance with the practice established between the parties, B's silence in regard to A's order amounts to an acceptance.
According to para. (2) an acceptance becomes effective at the moment the indication of assent reaches the offeror (see Art. 1.9(2)). For the definition of "reaches" see Art. 1.9(3). The reason for the adoption of the "receipt" principle in preference to the "dispatch" principle is that the risk of transmission is better placed on the offeree than on the offeror, since it is the former who chooses the means of communication, who knows whether the chosen means of communication is subject to special risks or delay, and who is consequently best able to take measures to ensure that the acceptance reaches its destination.
An exception to the general rule of para. (2) is to be found in the cases envisaged in para. (3), i.e. where "by virtue of the offer or as a result of practices which the parties have established between themselves or of usage, the offeree may indicate assent by performing an act without notice to the offeror". In such cases the acceptance is effective at the moment the act is performed, irrespective of whether or not the offeror is promptly informed thereof.
3. A asks B to write a special program for the setting up of a data bank. Without giving A notice of acceptance, B begins to write the program and, after its completion, insists on payment in accordance with the terms set out in A's offer. B is not entitled to payment since B's purported acceptance of A's offer never became effective as B never notified A of it.
4. The facts are the same as in Illustration 3, the difference being that in the offer B is informed of A's absence for the following two weeks, and that if B intends to accept the offer B should begin writing the program immediately so as to save time. The contract is concluded once B begins to perform, even if B fails to inform A thereof either immediately or at a later stage.
This article corresponds to para. (1), (2) first part and (3) of Art.18 CISG.
With respect to the time within which an offer must be accepted, this article, which corresponds to the second part of para. (2) of Art.18 CISG, distinguishes between oral and written offers. Oral offers must be accepted immediately unless the circumstances indicate otherwise. As to written offers, all depends upon whether or not the offer indicated a fixed time for acceptance: if it did, the offer must be accepted within that time, while in all other cases the indication of assent must reach the offeror "within a reasonable time having regard to the circumstances, including the rapidity of the means of communication employed by the offeror".
It is important to note that the rules laid down in this article also apply to situations where, in accordance with Art. 2.6(3) [see above], the offeree may indicate assent by performing an act without notice to the offeror: in these cases it is the act of performance which has to be accomplished within the respective periods of time.
For the determination of the precise starting point of the period of time fixed by the offeror, and the calculation of holidays occurring during that period of time, see Art. 2.8; as to cases of late acceptance and of delay in transmission, see Art. 2.9.

References: Art. 18
 Art. 18
 Art. 6

Art. 2
 Art. 18
 Art. 18
 Art. 18
 Art. 2
 Art. 2
 Art. 18
 Art. 2
 Art.18
 Art. 2
 Art. 11
 Art. 13

Art. 1
 Art. 13
 Art. 18
 Art. 18
 Art. 18
 Art. 18
 Art. 18
 Art. 16
 Art. 2

Art. 2
 Art. 24
 Art. 1
 Art. 1
 Art. 24
 Art. 21
 Art. 2
 Art. 18
 Art. 18
 Art. 23
 Art. 2
 Art. 18
 Art. 9
 v. 
 Art. 18
 Art. 2
 Art. 2
 Art. 2
 v. 
 Art. 18
 v. 
 v. 
 Art. 6
 Art. 18
 Art. 2
 Art. 2
 Art. 2
 Art. 2
 Art. 18
 art. 11
 Art. 13
 Art.1
 Art. 2
 Art. 1
 Art. 1
 Art. 2
 Art. 24
 Art. 1
 Art. 20
 Art. 2
 Art. 20
 Art. 2
 Art. 18
 Art. 21
 Art. 21
 Art. 2
 Art. 18
 Art. 2
 Art. 2
 Art. 1
 Art. 1
 Art.18
 Art.18
 Art. 2
 Art. 2
 Art. 2