Summarize the following court judgment. Civil Appeals Nos. 58 59 and 880 883 of 1971. From the Judgment and order dated 10 4 1970 of the Madras High Court in Writ Petition Nos. 437/67 and 520/68 and Tax Cases Nos. 135 138 of 1970 respectively. P. Ram Reddy	 A. V. Rangam and Miss A. Subhashini	 for the Appellant in C.As. 58 59/71. Sachin Chandra Chaudhury and Mrs. section Gopalakrishnan for Respondent. Gobind Das. P. H. Parekh and Miss Manju Jetley for the Intervener (M/s Durga Steel) The Judgment of the Court was delivered by BEG	 J. The two Civil Appeals Nos.58 59 of 1971 arise out of a judgment of a Division Bench of the Madras High Court dismissing two Writ Petitions filed against notices issued by a Commercial Tax officer showing institution of Sales tax assessment proceedings in respect of certain iron and steel goods for the assessment year 1965 66 in Writ Petition No. 437 of 1967 and for the assessment year 1966 67 in Writ Petition No. 520 of 1968. The High Court of Madras had certified the cases as fit for appeal to this Court under Article 132 and 133(1)(a) and (c) of the Constitution. Although	 the Writ Petitions had been dismissed on the ground that they involve an investigation into the question of fact whether the iron and steel scrap	 out of which the manufactured goods	 sought to be subjected to Sales tax	 had been made	 were already taxed or not	 yet	 the State of Tamil Nadu was aggrieved by the decision of the Madras High Court holding that the manufactured goods	 said to consist of "steel rounds	 flats	 angles	 plates	 bars" or similar goods in other forms and shapes	 could not be taxed again if the material out of which they were made had already been subjected to sales ' tax once an iron and steel scrap as both were "Iron and steel". It was possible to leave the assessing authorities free to decide all the questions which they had jurisdiction to consider. But	 it appears that the Madras High Court thought it proper to decide the question as the Sales ' tax 170 authorities had already adopted the view	 in other cases	 that such goods	 though covered by the broad genus "Iron and Steel"	 were separately taxable commodities because each kind of "Iron and Steel" goods was a commercially different and separately taxable species or category. Moreover	 this very question was also before the High Court in regular revision petitions under the Tamil Nadu Sales Tax Act (hereinafter referred to as 'the Tamil Nadu Act '). Civil Appeals Nos. 880 883 of 1971 arise out of four petitions for revision under the provisions of the Tamil Nadu Act for the years 1964 65 and 1965 66	 which were allowed by the Madras High Court 4 setting aside assessment orders by following its judgment and decision mentioned above given on 24 6 1970 on Writ Petitions Nos. 437 of 1967 and 520 of 1968. The Madras High Court had also granted Certificates of fitness for appeal to this Court under Article 132 read " with Article 133(1)(a)(c) in the four cases before it on revision petitions. Hence	 six cases were connected and heard together by us. 58 59 of 1971 before this Court	 arise in all of them. All the six cases before us relate to what are known as "declared goods" under Section 14 of the Central Sales Tax Act (hereinafter referred to as 'the Central Act '). It was claimed	 on behalf of the r dealers	 sought to be assessed in each case	 that	 by reason of the restrictions imposed by Section 15 of the Central Act	 the levy of tax under the Tamil Nadu Act was not permissible. The list of goods given there at No. (iv)	 as it stood in 1968	 was: "(IV) Iron and Steel	 that it to say (a) 'pig iron and iron scrap (b) iron plates sold in the same form in which they are directly Produced by the rolling mill; (C) steel scrap	 steel ingots	 steel billets	 steel bars and rods; (d) (i) steel plates (ii) steel sheets	 (iii) sheet bars and tin bars	 sold in the same form in which they are (iv) rolled steel sections	 directly produced by the rolling mill;" (v) tool alloy steel	 	 sole in the same from in which they are directly produced by the rolling mil;" By the Central Sales Tax (Amendment) Act 61 of 1972	 clause (iv) r was redrafted. It now reads as follows: "(iv) iron and steel	 that is to say (i) pig iron and cast iron including ingot	 moulds	 bottom plates	 iron scrap	 cast iron scrap	 runner scrap andiron skull scrap; (ii) steel semis (ingots	 slabs	 blooms and billets of all qualities	 shapes and sizes); (iii)skelp bars	 tin bars	 sheet bars	 heebars and sleeper bars; 171 (iv) steel bars (rounds	 rods	 squares	 flats	 octagone and A hexagone	 plain and ribbed or twister	 in coil from as well as straight lengths): (v) steel structurals (angles	 joints	 channels	 tees	 sheet piling sections	 sections or any other rolled sections); (vi) sheets	 hoops	 strips and skelp	 both black and galvanised	 hot and cold rolled	 plain and corrugated	 in all qualities	 in straight lengths and in coil form	 as rolled and in rivetted condition; (vii)plates both plain and chequered in all qualities; (viii)discs	 rings	 forgings and steel castings; (ix) tool	 alloy and special steels of any of the above categories; (x) steel melting scrap in all forms including steel kull	 turnings and borings; (xi) steel cubes	 both welded and seamless	 of all diameters and lengths	 including tube fittings; (xii)tin plates	 both hot dipped and electrolytic and tin free plates; (xiii)fish plate bars	 bearing plate bars	 crossing sleeper bars	 fish plates	 bearing plates	 crossing sleepers and pressed steel sleepers	 rails heavy and crane rails; (xiv)wheels	 tyres	 axles and wheel sets; (xv) wire rods and wires rolled	 drawn	 galvanised	 alumanised	 tinned or coated such as by copper; (xvi)defectives	 rejects	 cuttings or end pieces of any of the above categories"; It will be seen that "Iron and Steel" is now divided into 16 categories which clearly embrace widely different commercial commodities	 from mere scrap iron and left overs of processes of manufacturing to "wires" and "wheels	 tyres	 axles	 and wheel sets". Some of the enumerated items like "melting scrap" or "tool alloys" and "special steels" could serve as raw material out of which other goods are made and others are definitely varieties of manufactured goods. The G reason given	 in the statement of objects and reasons of the 1972 Act	 for an elucidation of the "definition" of iron and steel	 was that the "definition" had led to varying interpretations by assessing authorities and the courts so that a comprehensive list of specified declared iron and steel goods would remove ambiguity. The Select Committee	 which recommended the amendment	 called each specified category "a sub item" falling under "Iron and Steer '. Apparently	 the intention was to consider each "sub item" as a separate taxable commodity for purposes of Sales ' tax. Perhaps some items could overlap	 but no difficultly arises in cases before us due to this feature. As we have 172 pointed out	 the statement of reasons for amendment spoke of Section 14(iv) as a "definition" of "Iron and Steel". A definition is expected to be exhaustive. Its very terms may	 however	 show that it is not meant to be exhaustive. For example	 a purported definition may say that the term sought to be defined "includes" what it specifies	 but	 in `that case	 the definition itself is not complete. Although	 we have looked at the subsequent amendment of 1972 in order to find an indication of the original intention	 because subsequent history of legislation is not irrelevant	 yet	 we think that	 even if we confine our attention to Section 14 as it originally stood at the relevant time	 with which we are concerned in the cases before us	 the object was not to lay down that all the categories or sub items of goods	 as specified separately even before the amendment of 1972	 were to b viewed as a single saleable commodity called "Iron and Steel" for purposes of determining a starting point for a series of sales. The more natural and normal meaning of such a mode of listing special or declared kinds of goods seems to us to be that the object of specification was to enumerate only those categories of items	 each of which was to serve as a new starting point for a series of sales	 which were to be classed as "declared" goods. What we have inferred above also appears to us to be the significance and effect of the use of words "that is to say" in accordance with their normal connotation and effect. Thus	 in Stroud 's Judicial Dictionary	 4th Edn.5	 at page 2753	 we find: "THAT IS TO SAY. (1) 'That is to say ' is the commencement of an ancillary clause which explains the meaning of the principal clause. It has the following properties: (1) it must not be contrary to the principal clause: (2) it must neither increase nor diminish it; (3) but where the principal clause is general in terms it may restrict it: see this explained with many examples	 Stukeloy vs Butler Hob. 171"; The quotation	 given above	 from Stroud 's Judicial Dictionary shows that	 ordinarily	 the expression "that is to say" is employed to make clear and fix the meaning of what is to be explained or defined. Such words are not used	 as a rule	 to amplify a meaning while removing a possible doubt for which purpose the word "includes" is generally employed. In unusual cases	 depending upon the context of the words "that is to say"	 this expression may be followed by illustrative in stances. In Megh Raj & Anr.vs Allah Rakhia & Ors.(1) the words "that is to say. "	 with reference to a general category "land" were held to introduce "the most general concept" when followed	 inter alia	 by the words: "Rights in or over land. Both were wide classes. The object of using them or subject matter of legislation was	 obviously	 to lay down a wide power to legislate. But	 in the context of single point sales ' tax	 subject to special conditions when imposed on separate categories of specified goods	 the expression was apparently employed to specifically enumerate separate categories of goods on a given list. The purpose of such specification and enumeration in a statute dealing with sales ' tax at a single point in a series of sales would	 very naturally	 be to indicate the types of goods each of which would constitute at separate class for a series of sales. Otherwise	 the listing itself loses all meaning and would be without any purpose behind it. Learned Counsel appearing for an intervener argued that the chemical position of iron and steel affords a clue to the meaning of "Iron and Steel" as used in Section 14 of the Central Act. We are unable to agree that this could be what Parliament or any legislature would be thinking of when enumerating items to be taxed as commercial goods. The ordinary meaning to be assigned to a taxable item in a list of specified items is that each item so specified is considered as a separately taxable item for purposes of single point taxation in a series of sales unless the contrary is shown. Some confusion has arisen because the separate items are all listed under one heading "Iron and Steel". If the object was to make iron and steel taxable as a substance	 the entry could have been: "Goods of Iron and Steel." Perhaps even this would not have been clear enough. The entry to clearly have that meaning would have to be: "Iron and Steel irrespective of change of form shape or character of goods made out of them". This is the very unusual meaning which the respondents would like us to adopt. If that was the meaning	 sales ' tax law itself would undergo a change from being a law which normally taxes sales of "goods" to a law which taxes sales of substances out of which goods are made. We	 however	 prefer the more natural and normal interpretation which follows plainly from the fact of separate specification and numbering of each item. This means that each item so specified forms a separate species for each series of sales although they may all belong to the genus: ' 'Iron and Steel." Hence	 if iron and steel "plates" are melted t and converted into "wire" and then sold in the market	 such wire would only be taxable once so long as it retains its identity as a commercial goods belonging to the category "wire" made of either iron or steel. The mere fact that the substance or raw material out of which it is made has also been taxed in some other form	 when it was sold as a separate commercial commodity	 would make up difference for purposes of the law of sales ' tax. The object appears to us to be to tax sales of. goods of each variety and not the sale of the substance. Out of which they are made. As we all know	 sales ' tax law is intended to tax sales of different commercial commodities and not to tax the production or manufacture of particular substances out of which these commodities may have 174 been made. As soon as separate commercial commodities emerge or come into existence	 they become separately taxable goods or entities for purposes of sales ' tax. Where commercial goods	 without change of their identity as such goods	 are merely subjected to some processing or finishing or are`merely jointed together	 they may remain commercially the same goods which cannot be taxed again	 in a series sales	 so long as they retain their identity as goods of a particular type. In State of Madhya Bharat vs Hiralal(1) this Court held that a dealer	 who bought some scrap iron locally and imported some iron plates from outside and then converted the material into bars	 flats and plates	 by rolling them in his mills	 and then sold them	 was still entitled to exemption given to iron and steel from sales tax. But	 in that case	 the language of the provision giving the exemption justified this interpretation. The exemption was given to a sale by either an importer or a purchaser of "goods prepared from any metal other than gold or silver. In that context	 it had become necessary to examine whether the exemption from sales ' tax was meant for all goods made out of a particular sub stance	 or for goods as separate commercial commodities. This Court held that the raw material from which the goods were made was decisive for the purposes of the exemption given. This Court said (at p 315): "A comparison of the said two Notifications brings out the distinction between raw materials of iron and steel and the goods prepared from iron and steel while the former is exempted from tax	 the latter is taxed. Therefore	 iron and steel used as raw material for manufacturing other goods are exempted from taxation. So long as iron and steel continue to be raw materials	 they enjoy the exemption. Scrap iron purchased by the respondent was merely re rolled into bars	 flats and plates. They were processed for convenience of sale. The raw materials were only re rolled to give them attractive and acceptable forms. They did not in the process lose their character as iron and steel. The dealer sold 'Iron and steel ' in the shape of bars	` flats and plates and the customer purchased 'iron and steel ' in that shape. We	 therefore	 hold that the bars	 flats and plates sold by the assessee are iron and steel exempted under the Notification. " The law to be interpreted in Hiralal 's case (supra) was entirely different. In interpreting it	 this Court did observe that a mere change of the form of a substance excepted from sales ' tax did not matter. The language of the notifications involved there made it clear that the exemption was for the metal used. In the cases before us now the object of single point taxation is the commercial commodity and not the sub stance out of which it is made. each commercial commodity here becomes a separate object of taxation in series of sales of that commercial commodity so long as it retains its identity as that commodity. vs (1) [1966] 17 S.T.C. 313	 315. 175 The State of Punjab & ors.(1) where Subba Rao	 C.J. speaking for a Constitution Bench of this Court	 said at (p. 447). "Now coming to Civil Appeals Nos. 39 to 43 of 1965	 the first additional point raised is that when iron scrap is converted into rolled steel it`does not involve the process of manufacture. Before the High Court this contention was not pressed. That apart	 it is clear that scrap iron ingots undergo a vital change in the process of manufacture and are converted into a different commodity	 viz	 rolled steer sections. During the process the scarp iron loses its identity and becomes a new marketable commodity. " The process is certainly one of manufacture. It is true that the question whether goods to be taxed have been subjected to a manufacturing process so as to produce a new market able commodity	 is the decisive test in determining whether an excise duty is leviable or not on certain goods. No doubt	 in the law dealing with the sales tax	 the taxable event is the sale and not the manufacture of goods. Nevertheless	 if the question is whether a new commercial commodity has come into existence or not	 so that its sale is a new taxable event	 in the Sales ' Tax law	 it may also become necessary to consider whether a manufacturing process	 which has altered the identity of the commercial commodity	 has taken place. The law of sale tax is also concerned with "goods" of various descriptions. It	 therefore	 becomes necessary to determine when they cease to be goods of one taxable description and become those of a commercially different category and description. It appears to us that the position has been simplified by the amendment of the law	 as indicated above	 so that each of the categories falling under "Iron and Steel" constitutes a new species of commercial commodity more clearly new. It follows that when one commercial commodity is transformed into another	 it becomes a separate commodity for purposes of sales tax. We think that the Madras High Court had committed an error in applying Hiralal 's case (supra) to the decision of cases now before us which turns really on a correct interpretation of Section 14 of the Central Act. On the question now before us	 we approve of the reasoning adopted by a Division Bench of the Punjab High Court in Devgun Iron & Steel Rolling Mills vs State of PunJab(2). Section 15 of the Central Act places certain restrictions and conditions upon State enactments imposing Sales tax. It says: Every sales tax law of a State shall	 in so for as it imposes or authorises the imposition of a tax on the sale or (1) (1967) 20 S.T.C. 430 at 447. (2) (1961) 12 S.T.C. p. 590 176 purchase of declared goods	 be subject to the following restrictions and conditions	 namely: (a) the tax payable under that law in respect of any sale or purchase of such goods inside the State shall not exceed three per cent of the sale or purchase price thereof	. and such tax shall not be levied at more than one stage; (b) where a tax has been levied under that law in respect of the sale or purchase inside the State of any declared goods and such goods are sold in the course of inter State trade or commerce	 and tax has been paid under this Act in respect of the sale of such goods in the course of inter State trade or commerce	 the tax levied under such law shall be reimbursed to the person making such sale ill the course of inter State trade or commerce in such manner and subject to such condition as may be provided in any law in force in that State". It has not been shown to us that any provision of the Tamil Nadu Sales Tax Act violates Section 15 of the Central Act enacted in accordance with Article 266(3) of the Constitution. Section 3 of the Tamil Nadu Act leviesm taxes on sales and purchases of "goods" as defined in Section 2(j) of the Act: "(j) 'goods	 means all kinds of movable property (other than newspapers	 actionable claims	 stocks and shares and securities) and includes all materials	 commodities	 and articles (including these to be used in the fitting out	 improvement or repair of movable property)	 and all growing crops grass or things attached to	 or forming part of	 the land which are agree to be severed before sale or under the contract of sale	 Section 4 of the Tamil Nadu Act lays down: 4. Tax in respect of declared goods. Notwithstanding anything contained in Section 3	 the tax under this Act shall be payable by a dealer or the sale or purchase inside the State of declared goods at the rate and only at the point specified against each in the Second Schedule on the turn over in such goods in each year	 whatever be the quantum of turnover in that year". Item 4 of the second schedule specifies the rates of tax in accordance with the Central Act. It reproduces Section 14(iv) of the Central Act. On an amendment of Section 14(iv) of the Central Act	 serial No. 4 of the second schedule of the Tamil Nadu Act was also correspondingly amended so as to reproduce the sixteen items found in Section 14(iv) of the Central Act. Other provisions only fortify our conclusion. 177 The result is that we allow these appeals. We set aside the orders of the High Court and restore the orders of the assessing authorities in cases giving rise to Civil Appeals Nos. 880 883 of 1971. In cases but of which Civil Appeals Nos.58 59 of 1971 arise	 we set aside the judgment of the High Court but maintain its order dismissing the Writ Petitions and order that the assessing authorities will now proceed to determine such question of fact and law as still survive for determination after the decision given above of the question considered by us. The parties will bear their own costs. P.H.P. Appeals allowed.

The respondents are dealers under the Tamil Nadu Sales Tax Act. Section 14 of the Central Sales Tax Act declares certain goods enumerated therein of ` ' special importance in inter state trade or commerce. The list of goods given at serial No. IV reads as under: (IV) Iron and Steel	 that is to say (a) pig iron and iron scrap . (b) iron plates sold in the same form in which they are directly produced by the rolling mill; (c) steel scrap	 steel ingots	 steel billets	 steel bars and rods	 (d) (i) steel plates (ii) steel sheets	 (iii) sheet bars and tin bars	 (iv) rolled steel sections	 I mill. (v) tool alloy steel	 J sole in the same from in which they are directly produced by the rolling mill. The said clause IV was amended by the Central Sales Tax Amendment Act	 Act 61 of 1972 by which certain more entries were added. Section 15 of the Central Sales Tax Act provides that the tax payable under a State Law on sale or purchase of declared goods shall not be levied at more than one stage. Respondents used to purchase iron scrap and thereafter used to convert them into steel rounds	 flats	 plates etc. The scrap was already subject to tax once. The respondents contended that the entry Iron & Steel was wide enough to include scrap as well as the steel rounds	 flats	 plates	 etc. made out of the scrap which was subject to tax once and that	 therefore	 the sale of the steel rounds	 flats	 plates	 etc.	 cannot be subjected to tax again under the Tamil Nadu Sales Tax Act. The High Court accepted the contention of the respondents. Allowing an appeal by certificate	 ^ HELD: 1. The intention was to consider each sub item in clause IV as a separate taxable commodity for purposes of sales tax. The object was not to lay down that all the categories or sub items of goods specified separately were to be viewed as a single saleable commodity called iron and steel for purposes j of determining a starting point for a series of sales. The note against sub division of Clause IV makes it clear that even each sub category of a sub item retains its identity as a commercially separate item for purposes of sales tax so long as it retains the sub division. [171Gm	 172B C] 2. The expression 'that is to say ' is employed to make it clear and fix the meaning of words to be explained or defined. Such words are not used	 as a rule	 to amplify a meaning while removing a possible doubt for which purpose the word 'includes ' is generally employed. The precise meaning of the words 169 that is to say ' must vary with the context. The purpose of the expression in a sales A lax law would be to indicate the types of goods each of which would constitute separate class for a series of sales. [172F H	 173B] 3. The chemical composition of iron and steel cannot afford a clue to the meaning of iron and steel. Sales Tax Law taxes sales of goods and is not a taxation on sales of substance out of which goods are made. We prefer to follow the more natural and normal interpretation which follows plainly from the fact of separate specification numbering of each item. State of Madhya Pradesh vs Hira Lal; (1966) 17 STC 313 315 distinguished. The case cf Devidas applied. [173C. E F] 4. It has not been shown to	 us that any provision of the Tamilnadu Sales Tax Act violates section 15 of the Central Sales Tax Act enacted in accordance with Article 286(3) of the Constitution. Section 2(j) of the Tamil Nadu Act defines goods and section 4 imposes charge in respect of tax on declared goods. The Tamilnadu Act borrows clause (IV) of section 14 of the Central Sales Tax Act. [176 C H] C