# Glossary of sound laws in the Indo-European languages

The Indo-European language family comprises a vast number of languages and dialects spoken throughout the world today. All of these languages are descended from a common ancestor known as Proto-Indo-European, which scholars estimate was spoken about six thousand years ago. This common ancestor has been reconstructed by historical linguists using the comparative method. Although there is disagreement about the historical relationship of these languages to each other, this glossary uses the neo-traditional model of Indo-European phylogeny which states the main branches of the family are Albanian, Anatolian, Armenian, Balto-Slavic, Celtic, Germanic, Hellenic, Indo-Iranian, Italic, and Tocharian.
This glossary provides a list of sound laws that have been formulated by linguists for the various Indo-European languages. Any sound laws which affects any of the major branches of the Indo-European family or more than one descendant language are included.

## Proto-Indo-European or multiple branches
### asno law

    The word-medial sequence *-mn- is simplified after long vowels and diphthongs or after a short vowel if the sequence was tautosyllabic and preceded by a consonant. The *n was deleted if the vocalic sequence following the cluster was accented, as in Ancient Greek θερμός (thermós 'warm') from Proto-Indo-European *gʷʰermnós ('warm'); otherwise, the *m was deleted, as in Sanskrit अश्नः (áśnaḥ) from Proto-Indo-European *h₂éḱmnes ('anvil [gen. sg.]'). The sequence remains if the *-mn- sequence is heterosyllabic, such as in Ancient Greek πρύμνος (prýmnos 'prominent'). The law was first discovered by Johannes Schmidt in 1895 and is named for the Avestan reflex 𐬀𐬯𐬥𐬋 asnō.

### aspirate throwback

    (Ancient Greek, Sanskrit) Also, aspiration throwback. When a root-final aspirated stop loses its aspiration for whatever reason, typically due to another process, the aspiration is retracted to the initial consonant whenever that initial consonant is capable of taking an aspirated quality. One example includes the Ancient Greek root τρίχ- (tríkh- 'hair'), which becomes θρίξ (thríx) in the nominative form. The process was mentioned earlier by the Sanskrit scholar Pāṇini, but brought to modern scholarship in the first clause of a two-part law proposed by Hermann Grassmann in 1863, though the name "aspirate throwback" appears later. The second clause is now referred to alone as Grassmann's law.

### Bartholomae's law

    Also, Buddha rule. If a cluster of two or more obstruents contains at least one voiced aspirated consonant, the whole cluster becomes voiced and aspirated. The process may have been inherited from Proto-Indo-European, though this is not universally accepted. The law is named after the German linguist Christian Bartholomae who discussed outcomes of the process in the various Indo-Iranian languages in 1882. The alternative name stems from the fact that the etymon for the Sanskrit word बुद्ध (buddhá 'awake, enlightened') is affected by this process, derived from *bʰewdʰ, meaning 'to be awake', and *-tó-, the passive past participle suffix.

### boukólos rule

    Labiovelars lose their labialization and become plain velars when preceded or followed by *w or *u. This dissimilatory process explains the reflex in words like Ancient Greek βουκόλος (boukólos 'cowherd'), derived from Proto-Indo-European *gʷoukʷólos. The expected form *βουπόλος (*boupólos) does not appear because the initial *kʷ in *kʷólos is preceded by the *u in *gʷou-, whereas in αιπόλος (aipólos 'goatherd'), the expected form -πόlos (-pólos) is attested, derived from Proto-Indo-European *ai(ǵ)kʷólos. This process remained productive into Proto-Germanic, where it also came to apply to labiovelars preceded by *-un- through an assimilatory process which caused *n to have a labialized allophone. Examples of this include Proto-Germanic *tungōn- from Proto-Indo-European *dn̥ǵʰwéh₂- both meaning 'tongue', whence both Latin lingua (from Old Latin dengua) and English tongue (from Old English tunge).

### Dybo's law

    Laryngeal consonants are lost between a vowel and any other consonant in pretonic syllables. Examples of this include Proto-Celtic *wiro- (whence Old Irish fer, meaning 'man'), Latin vir ('man'), and Old English wer ('man'), all of which are derived from Proto-Indo-European *wiHró-. If the vowel is long before the process occurs, it is shortened. The process likely did not take place in Proto-Indo-European as it only affected the western Indo-European languages; examples from eastern families show laryngeal retention, such as Lithuanian výras ('man, husband') and Sanskrit वीरः (vīráḥ 'man, hero'). The law is named for Vladimir Dybo, who published work on the topic in 1961.

### Grassmann's law

    (Ancient Greek, Sanskrit) Also, ha-ha rule; breath dissimilation. When an aspirated consonant is followed by another aspirated consonant in same or following syllable, the first consonant loses its aspiration. Examples of pairs affected by this process include Ancient Greek θρίξ (thríx 'hair') in the nominative case, but τριχός (trikhós) in the genitive case. Hermann Grassmann first proposed this process in 1863 as the second clause of a two-part law. The first clause is now known as the aspirate throwback. The law remained productive after the Greek devoicing of aspirates and /h/, from earlier *s, which behaved as an aspirated stop.

### Kortlandt effect

    Proto-Indo-European *d undergoes debuccalization, becoming the laryngeal *h₁, whenever it is followed by a dental consonant, a consonant followed by a dental, or whenever the following syllable begins with a dental. Examples include Ancient Greek ἑκατόν (hekatón 'one hundred') from *dḱm̥tom, where the simple loss of the initial *d is attested in forms like Latin centum and Sanskrit शतम् (śatám), but the debuccalization of *d instead of deletion adequately explains the initial vowel in the Greek term. The process also explains the relationship between the Ancient Greek terms δρέπω (drépō 'I pluck'), from the Proto-Indo-European root *drep-, and Homeric Greek *ἐρέπτομαι (*eréptomai 'I feed on, I munch'), from the same root affected by the debuccalization (*h₁rep-). The law is named after Frederik Kortlandt, who first proposed the sound change in 1983. The debuccalization appears to have taken place before the Anatolian languages split off from Proto-Indo-European.

### Kuiper's law

    Laryngeals are lost in utterance-final position. The process does not cause compensatory lengthening. The Latin nominative singular, which ends in -a rather than the expected *-ā, is thought to be derived from an earlier usage of the vocative case, which would have invoked the law; this process did not affect the related Sabellic languages, however. The process similarly affected the vocative in Ancient Greek as well. The law is named for the Dutch linguist F. B. J. Kuiper, who published work on this process in 1955.

### *kʷetwóres rule

    In a word of three syllables with a vowel pattern *é-o-V, where V is any vowel, the accent is moved forward to the middle syllable, becoming *e-ó-V. This explains the penultimate accent in terms like Vedic Sanskrit चत्वारः (catvā́raḥ), the nominative plural form of 'four', from Proto-Indo-European *kʷetwóres. The law is named after this example.

### métron rule

    In a dental–dental–resonant sequence, one of the dental consonants is deleted and there is no compensatory lengthening. Evidence from Latin and Gaulish seems to suggest that the second dental consonant was simply deleted, but evidence from Greek and Sanskrit indicates that the second dental consonant underwent voicing assimilation and then the resulting geminate was shortened; Proto-Germanic evidence provides examples of both. The name is derived from the Greek term μέτρον (métron 'measure'), derived from Proto-Indo-European *méd-tro-.

### neognós rule

    Laryngeals are lost in zero-grade contexts where full-grade root contains a consonant–vowel–resonant–laryngeal string, in that order, in certain reduplicated forms and in some other compounds. Examples include the Ancient Greek term νεογνός (neognós 'newborn'); the Greek term is derived from Proto-Indo-European *newoǵn̥h₁o- through the medial form *newoǵno-.

### Osthoff's law

    When a long vowel is followed by either a liquid or nasal consonant which is itself followed by a stop consonant or *s, the vowel is shortened. In addition to liquids or nasals, glides may also trigger the process in that position, though this is controversial. The law is named after Hermann Osthoff, who first postulated the process in 1879, followed by two important reanalyses in 1881 and 1884, though this particular appellation was not universal until as late as 1939.

### Pinault's law

    Also, Pinault's rule. Laryngeals are dropped in word-medial position between a consonant and *y, such as in Latin socius 'friend' from Proto-Indo-European *sokʷh₂-yo-. Only *h₂ and *h₃ appear to have been affected, though the law has been invoked to explain instances of *h₁'s disappearance in the same context, such as Proto-Celtic *gan-yo- 'to be born' from Proto-Indo-European *ǵnh₁yetor. The law is named for the French linguist Georges-Jean Pinault.

### ruki sound law

    (Balto-Slavic, Albanian, Armenian, Indo-Iranian) Also, ruki rule; ruki change; iurk rule; Pedersen's law. *s is retracted to a postalveolar *š when preceded by *r, *u, *k or *i. This also includes *g and *gʰ, which are devoiced before *s, and also the allophones *r̥, *w, *y. But it does not include the palatovelar *ḱ. In Indo-Iranian, it is also triggered by *l, which merges with *r. In Slavic, *š is further retracted to *x unless followed by a front vowel or *j. The name is derived from the constitutent letters which govern the change. Its order is a mnemonic device; ruki (Russian: руки) means 'hands'. Although the concept was articulated as early as 1818 by Rasmus Rask, the alternative name is derived from Holger Pedersen, who wrote about the process in detail.

### Siebs's law

    If an s-mobile is added to a root that begins with a voiced consonant, that consonant is devoiced. If it is aspirated, it retains its aspiration.

### Stang's law

    Word-finally, when a laryngeal, *y or *w is preceded by a vowel and followed by a nasal consonant, it is dropped and the preceding vowel is lengthened.

### Szemerényi's law

    1. (Not fully accepted) In pre-Proto-Indo-European, the word-final fricatives *s and *h₂ are deleted following a vowel–resonant sequence, followed by compensatory lengthening. Some linguists include in the law a process where if the resulting sequence is *-ōn, the *n is also dropped, but others describe that deletion as a separate process. The law is named after the Hungarian-British linguist Oswald Szemerényi who first described the process in 1956.
    2. Also, broad Szemerényi's law; final Szemerényi's law. In pre-Proto-Indo-European, syllable-final fricatives are deleted following a vowel–consonant sequence, followed by compensatory lengthening. The process is blocked if the consonant preceding the fricative is in a cluster or if the following syllable begins with a consonant.

### weather rule

    Laryngeals are lost in word-medial position preceding a stop followed by a resonant and a vowel. The law is named for its reflex in English, weather, which is derived from Proto-Indo-European *weh₁dʰrom ('weather'); the laryngeal *h₁ is deleted before the sequence *-dʰro- which comprises a stop, a resonant, and a vowel, respectively. The law does not cause compensatory lengthening.

### Weise's law

    Palatovelar consonants *ḱ *ǵ *ǵʰ are depalatalized when preceding *r unless that *r is followed by an *i. The law is named after the German linguist Oskar Weise who observed the results of the change in a 1881 essay on the topic.

## Albanian
### Rosenthall's law

    Within a morpheme, only one nasal–stop cluster is allowed; if more than one cluster exists in the underlying representation, any of these clusters created by morphophonological processes, such as epenthesis, are deleted. Examples are found in the terms kuvend ('assembly') and mbrëmë ('last night'), where the respective expected epenthetic forms *nguvend and *mbrëmbë reduced following this process. The process has been likened to Lyman's law in Japanese and Grassmann's law in Ancient Greek and Sanskrit. The law is named for the American linguist Samuel Rosenthall who first proposed the process in 2022.

## Armenian
### Adjarian's law

    Also Acharian's law. In some Armenian dialects, the vowels in an initial syllable are fronted after voiced stops. The process appears to have been mediated by the advancement of the tongue root, evidenced by some dialects in Malatya among others. The law is named after Hrachia Acharian, who first described the process in 1901.

## Balto-Slavic
### Lidén's law

    Word-initial *w in Proto-Indo-European is lost before non-syllabic *r and *l as the language developed into Proto-Balto-Slavic. The law is named for the Swedish linguist Evald Lidén [sv; no; hy], who wrote about the process in 1899. The process described by the law likely occurred after the development of Hirt's law, but before the syllabification of resonants. While it is possible that the law occurred after the syllabification of resonants and only affected non-syllabic resonants, Ranko Matasović finds this "improbable on phonetic grounds".

### Hirt's law

    Also, Hirt–Illich-Svitych's law. If the syllable preceding the expected stressed syllable has a vowel immediately followed by a laryngeal, the stress is retracted to that syllable. Examples include comparisons of Lithuanian výras ('man, husband') and mótė ('mother') with Sanskrit वीरः (vīráḥ 'man, hero') and माता (mātā́ 'mother'), respectively.The law was first proposed by the German philologist Hermann Hirt in 1895, but the original formulation was corrected in 1963 by the Soviet linguist Vladislav Illich-Svitych. The law applies to the original PIE accent placement, but after leveling of PIE mobile-accented paradigms into end-stressed paradigms. It also applied before the epenthesis before syllabic sonorants.

### Pedersen's law

    In words with a Balto-Slavic mobile accent paradigm, the accent was retracted from a medial onto the initial syllable. In Proto-Slavic, a similar analogical change caused the retraction of the accent onto a preceding unaccented clitic, such as a preposition.

### Winter's law

    Short vowels with non-acute accents are lengthened before unaspirated voiced stops (*b, *d, *g, but not *ǵ). The newly lengthened vowel receives the acute accent. The law is named after the German linguist Werner Winter who wrote his proposal in 1976, though it was not published until 1978. Frederik Kortlandt has dated the law to the final years of the Balto-Slavic period.

### Baltic
#### de Saussure's law

    (Lithuanian) Also, Saussure's law. If a non-acuted accented syllable is followed by an acuted syllable, the accent shifts forwards onto the acuted syllable. This split the Balto-Slavic fixed accent paradigm into Lithuanian paradigms 1 and 2, and the mobile accent paradigm into paradigms 3 and 4.

#### Hjelmslev's law

    (Lithuanian) When a vowel receives an accent, it takes on the intonation of the following syllable. The law is named for the Danish linguist Louis Hjelmslev, who proposed the process in his doctoral thesis in 1932. Frederik Kortlandt states that, if the ictus retracts to a laryngealized vowel, the laryngeal is deleted and the result is a rising tone, but N. E. Collinge argues this is beyond the scope of process and that both Hjelmslev and de Saussure allow for a falling tone in their analyses.

#### Leskien's law

    (Lithuanian) If a word-final long vowel contains a falling accent, it is shortened. This process precedes Hjemslev's law, but is preceded by de Saussure's and Nieminen's laws. The law is named for August Leskien, who first established the law in 1881.

#### Nieminen's law

    When the Proto-Baltic sequence *-ás is found word-finally, it is reduced to *-əs and loses its ability to carry stress. As a result, final stress is retracted to the penultimate syllable. This change explains the accentuation paradigm of o-stem nouns. Examples include báltas ('white') and var̃nas ('raven') from earlier *langás and *warnás, respectively. This is contrasted with mobile paradigms, where the accent is final, such as in galvà 'head' and žvėrìs 'beast'. The law is named after the Finnish linguist Eino Nieminen [fi; lt; eo; pl] who proposed the law in 1922. The process certainly precedes Leskien's law in Lithuanian, but has been suggested to go at least as far back as Proto-Baltic based on evidence from Old Prussian.

### Slavic
#### Dolobko's law

    Also, Vasilyev–Dolobko's law. In a word with mobile stress and a final encliticized morpheme, the stress moves from the initial syllable to the final syllable. If the encliticized morpheme has no syllable, the stress moves to the preceding syllable. This process occurred sometime between the end of the Balto-Slavic period and the loss of intervocalic *j, which places it sometime before Dybo's law. The law is named after Mily Dolobko [ru] who wrote about the law in 1927. Because its first formulation was by L. L. Vasilyev [ru] in 1905, it is sometimes known by the joint name.

#### Dybo's law

    Also, Dybo–Illich-Svitych's law. If a syllable was non-acute and accented, the accent was advanced onto the following syllable. The originally accented syllable retains its length. The change was prevented if the word had a mobile accent paradigm.

#### fall of the weak yers

    Following the results of Havlík's law, weak yers were deleted. Examples are found in Old East Slavic, where earlier forms for the word 'prince' demonstrate a retained yer (кънѧзь [kъnjazь]), but later forms show it without (кнѧзь [knjazь]); the retained forms are dated to around 1075, while loss became widespread between the 1120s and 1210s.

#### Havlík's law

    In late Proto-Slavic, the vowels represented by *ь and *ъ, called yers, become "weak" – that is, subject to moraic shortening – in final position or if they are followed by a vowel other than another yer. A yer becomes "strong" if the following syllable contained a weak yer. In words with three successive syllables all containing yers, the final yer was weak, causing the penultimate syllable to be strong and the antepenultimate weak. This process represents the first phase of the larger yer shift, where the second phase is known as the fall of the weak yers, where weak yers were deleted.

#### Ivšić's law

    Also, Stang's law; Stang–Ivšić's law. Accented weak yers, as according to Havlík's law, lost their accent to the preceding syllable, which received a "neoacute" accent.

#### law of open syllables

    Also, opening of syllables; tendency to rising sonority. Word- and syllable-final obstruents and obstruent clusters are deleted. Finals nasals are lost after short vowels and nasalized after long vowels.

#### Meillet's law

    In words with a mobile accent paradigm, if the first syllable is accented with a rising (acute) accent in Proto-Balto-Slavic, it is converted into a falling (circumflex) accent in Proto-Slavic. Examples of this include Serbo-Croatian gláva in the nominative case, but glȃvu in the accusative. The process appears to be somewhat resilient to analogical leveling. The law is named after the French linguist Antoine Meillet who first described the law in 1902.

#### Shakhmatov's law

    (No longer widely accepted) Also, Šaxmatov's law. Stressed short falling (circumflex) accents shift to the preceding syllable. The law is named after Aleksey Shakhmatov who described the process in 1915.

#### Slavic first palatalization

    Also, first palatalization of velars; first regressive palatalization. When the velar sounds *k, *g, and *x occur before front vowels and *y, they undergo palatalization and become *č, *ž, and *š, respectively. If the velar follows a sibilant (i.e., *sk or *zg), the sibilant was backed to *šč and *ždž before the cluster was reduced in some dialects to *št and *žd. The process was probably mediated by coronalization before a final assibilation. The time at which the process occurred is unknown but it must have begun prior to the Migration Period and was completed sometime between the 6th and 8th centuries.

#### Slavic second palatalization

    Also, second palatalization of velars; second regressive palatalization. Following the monophthongization of Proto-Slavic *ai to *ě in open syllables and *i in closed syllables, velars became palatalized again. The process had different outcomes based on dialect: *k typically became *c and *g became either *dz or simply *z, but *x became *š in West Slavic, *xʲ in Novgorodian, and *s elsewhere. The process is estimated to have happened around the same time the West Slavs were migrating north of Carpathia and the Novgorodians were migrating to what is now northwestern Russia. Like the first palatalization, the process occurred in several stages: palatal coarticulation, palatalization, and assibilation. It appears that Novgorodian only went through the first stage. Clusters that had originally contained *ku̯ underwent palatalization in South Slavic and some East Slavic unless it was preceded by *s, but West Slavic and Novgorodian did not experience this palatalization at all.

#### Slavic third palatalization

    Also, progressive velar palatalization; palatalization of Baudouin de Courtenay. When Proto-Slavic *i, *ī, or *in precede a velar, the velar is palatalized and then assibilated; *k and *g become *c and *dz, respectively, in all languages, with *dz undergoing further lenition to *z outside of Eastern South Slavic, Slovak, and Lechitic. In West Slavic, *x became *š, but *s or *sʲ in South and East Slavic. Examples of this process include earlier *liːkad, meaning 'face', which became *liːca (whence Old Czech líce), and *mεːsinkaːd, the genitive singular form of 'moon', which became *mεːsĩːcaː (whence Old Polish miesięca). The first stage of this process was in effect by the 6th and 7th centuries.

#### Stang's law

    If a word-final syllable was long falling (circumflex) accented, the accent was retracted onto the preceding syllable. The originally accented syllable is shortened, and the newly-accented syllable receives a "neoacute" accent. This change applied after Dybo's law, and often "undid" it by shifting the accent back again.

#### van Wijk's law

    Short vowels, except for yers (*ь, *ъ) and nasal vowels, are lengthened when preceded by a palatal consonant. Although Aleksey Shakhmatov was the first to suggest the process in 1898, the law is named for the Dutch linguist Nicolaas van Wijk, who published work on the process in 1916.

#### yer shift

    Also, jer shift; third Slavic vowel shift; fall of the yers. The yer vowels, *ь and *ъ, underwent two-part process followed by a third step that fractured the realization of the vowel qualities. First, yer vowels underwent an alternating pattern of weakening every other yer in a word beginning with weakening the final one; this first part of the shift is referred to as Havlík's law. The process is a patterned form of compensatory lengthening. Next, the weak yers were deleted, referred to as the fall of the weak yers. Following this deletion, the remaining strong yers were thereby shifted to different vowel qualities in the various Slavic languages, which collectively are known as the vocalization of the yers.

## Celtic
### Joseph's law

    During the period between Proto-Indo-European and Proto-Celtic, when *e is followed by a resonant then by *a, the *e assimilates to *a. In other words, in the sequence *eRa, where *R signifies any resonant, *e becomes *a, thereby becoming *aRa. Examples of this change include Proto-Celtic *taratro- ('drill'), whence Irish tarathar ('auger') from earlier *teratro-, derived from Proto-Indo-European terh₁tro-, whence Ancient Greek τέρετρον (téretron 'borer, gimlet'). The law does not affect *ā and probably did not affect environments where the *a was word-final. The law also appears to have affected words where the *a was formerly a laryngeal consonant in Proto-Indo-European, such as in Proto-Celtic *banatlo- ('broom plant') which may be derived from Proto-Indo-European *bʰenH-tlo from a root meaning 'to hit, to strike'. The process was expanded in Welsh include environments where the resonant is followed by a nasal, explaining the vowel quality in Welsh words like sarnu 'to trample' but not Old Irish sernaid 'to arrange, to order', both from a Proto-Celtic *sternū/*starnati paradigm (from an older subjunctive Proto-Indo-European form *ster-nh₂-e/o, cognate with Latin sternō). Similarly, another expansion of the process appears in both Brittonic and Gaulish, causing the assimilation of *o to *a in the resonant–vowel environment *oRa, thereby rendering *aRa. Compare Middle Welsh taran ('thunder') and Gaulish Taranis ('the Celtic god of thunder'), which were affected by the expanded law, with Old Irish torann ('thunder'), which was not. The law is named after Lionel Joseph who covered the topic in 1982.

### MacNeill's law

    (Old Irish) Also, MacNeill–O'Brien's law. Lenition of Old Irish n, r, and l is lost in final unstressed syllables even though they are etymologically expected to be lenited in that position.

### McCone's law

    1. Proto-Celtic *b and *u̯ become *β before *n word-internally. The latter change is rare, but occurs in words like Old Irish amnair ('maternal uncle') from Proto-Celtic *au̯n and omun ('fear') from *ɸoβnos. The law appears to have only occurred in contexts where there is no front vowel preceding the cluster, which accounts for apparent counterexamples like Welsh clun ('hip, haunch') from Proto-Indo-European *ḱlownis and Old Irish búan ('permanent') from *bʰewHnos, though some other etymologies account for different sources; the Welsh term may be a later borrowing from Latin clūnis, for example. The law shares a relationship to another Proto-Celtic sound change where *ɸ became *u̯ between either *a or *o and *n.
    2. (Old Irish) Unless the syllable is stressed, voiceless obstruents are voiced word-initially and word-finally.

## Germanic
### Cowgill's law of Germanic

    (Not fully accepted) When preceded by a sonorant and followed by *w, the PIE laryngeal *h₃ (and possibly also *h₂) appears as *k in Proto-Germanic after the application of Grimm's law.

### Germanic spirant law

    Also, Primärberührung. When a plosive is followed by any voiceless sound, normally *s or *t, it becomes a voiceless fricative and loses its labialization if present. A dental consonant followed by *s becomes *ss, and followed by *t becomes either *ss (in older inherited forms) or *st (in newer and productive forms).

### Grimm's law

    Also, first Germanic sound shift; first Germanic consonant shift; Rask's rule. The three series of Proto-Indo-European plosives undergo a chain shift. The first shift causes voiceless stops – *p, *t, *k, and *kʷ – to become the voiceless fricatives *f, *θ, *x, and *xʷ, respectively. Next, the plain voiced stops – *b, *d, *g, and *gʷ – devoice and become *p, *t, *k, and *kʷ, respectively. Lastly, the aspirated voiced stops – *bʰ, *dʰ, *gʰ, and *gʷʰ – become voiced stops *b, *d, *g, and *gʷ, respectively. This sound change is sometimes obfuscated in Old High German as a result of the High German consonant shift. The process did not affect the second consonant in a cluster of two adjacent obstruents. Compare two versions of the Old Frisian word for 'throat': strot- and throt-. Both are derived from the Proto-Indo-European s-mobile root *(s)trewd-, the former including the s-mobile while the latter does not. In this example, the form with the s-mobile blocks the assibilation of the *t. Several other exceptions are covered by Verner's law. The Danish linguist Rasmus Rask is credited with first articulating the law in 1818, but its more common name is from German folklorist Jacob Grimm, who – after reading Rask's work – expanded on it and published it in the preface of his German grammar book in 1819 with a rewrite in 1822.

### Holtzmann's law

    (North and East Germanic) Also, Verschärfung; sharpening; intensification. The geminated glides *jj and *ww are hardened into geminate plosives. In North Germanic, *jj becomes *ggj, while it became *ddj in East Germanic, *ww becomes *ggw in both.

### Ingvaeonic nasal spirant law

    (Ingvaeonic languages) When *n is followed by a fricative, the *n is lost and the preceding vowel is lengthened to compensate. The vowel was probably nasalized first and then experienced a loss in nasal quality before lengthening.

### Kluge's law

    (Not generally accepted) PIE plosives merge with a following *n and become geminate voiceless plosives in Germanic.

### Sievers's law

    Suffixal *j alternates with *ij depending on the syllable weight (length) of the preceding morpheme. *j appears after "light" or "short" morphemes, which consist of a single syllable ending in a short vowel and a single consonant. *ij appears elsewhere, including all morphemes with more than one syllable.

### Thurneysen's law

    (Gothic) Spirants in unaccented syllables changed their voiced–unvoiced quality based on the quality of the preceding consonant, whereby voiced spirants appear after unvoiced consonants and voiceless spirants appear after voiced consonants. Examples of both can be found in the dative singular forms, 𐌰𐌲𐌹𐍃𐌰 agisa 'fear' and 𐍂𐌹𐌵𐌹𐌶𐌰 riqiza 'darkness'. In short, spirants are voiced when they are immediately preceded by a vowel without primary stress and the preceding consonant before the vowel is unvoiced. If the preceding consonant is a cluster where the second consonant is a liquid, the spirant remains unvoiced, but if the second consonant is a glide, it is voiced. The process does not affect word-final spirants and the second element in a compound word where the simplex is stress-bearing. While some exceptions occur due to morphological leveling, there are at least seven words for which Thurneysen could not supply an explanation. The law is named after the Swiss linguist Rudolf Thurneysen who posited the law in 1896 and published it in 1898.

### Verner's law

    After the application of Grimm's law, when a voiceless fricative is preceded by an unaccented syllable, it is voiced (*f, *þ, *h, *hʷ, *s > *b, *d, *g, *gʷ, *z). Following this, the mobile PIE accent is lost and all words receive stress on the first syllable.

## Greek
### Bartoli's law

    Also, Bàrtoli's law. In short–long metrical feet where oxytone stress is expected, the syllable becomes paroxytone before a word boundary. The law only occurs in anapestic (short–short–long) and cretic (long–short–long) contexts. An example of this process can be seen in Ancient Greek θυγάτηρ (thygátēr 'daughter'), in its nominative singular form, when contrasted with its accusative singular form θυγατέρα (thygatéra) and with the Sanskrit nominative singular दुहिता (duhitā́ 'daughter'). While some exceptions can be attributed to analogical change, there are still some unexplained exceptions. The law is named after the Italian linguist Matteo Bartoli, whose 1930 publication contrasted Sanskrit oxytone words with their Ancient Greek paroxytone cognates.

### Cowgill's law of Greek

    Whenever Proto-Indo-European *o occurs between a resonant and a labial, it becomes Greek υ y. The resonant and the labial can be on either side of the *o and produce the same output. Examples of a resonant followed by *o followed by a labial include νύξ (nýx 'night') from *nokʷt-, whence also Latin nox 'night'. This law is preceded by laryngeal coloring, meaning that Proto-Indo-European sequences of *h₁o, *h₃o, and *h₃e are also accounted for in the law, as are cases in which the zero-grade form was vocalized, such as in στόρνυμεν (stórnymen 'we smooth out') from Proto-Indo-European *str̥-n-h̥₃-, showing the nasal infix. Some later sound changes obfuscate the law, but there is evidence to show that the sound change still occurred. For example, Proto-Indo-European *nomn̥ ('name') gave way to Proto-Greek *onuma. Although in Attic Greek the form became ὄνομα (ónoma), the expected form ὄνυμα (ónyma) is found in both Doric and Aeolic Greek; the expected form is also found in derivatives such as ἀνώνυμος (anṓnymos 'nameless, inglorious').

### Vendryes's law

    Also, Vendryès's law. Any perispomenon with a short vowel in the antepenultimate becomes proparoxytone in Attic. The law is named after the French linguist Joseph Vendryes.

### Wheeler's law

    Also, law of dactylic retraction. Oxytone words in Proto-Indo-European become paroxytone in Ancient Greek if the word has a dactylic ending; in other words, the stress in dactylic words moved from the final syllable to the penultimate syllable. The law counts endings such as ον -on, ος -os, and οι -oi as short. Examples of this tonic retraction include cognate pairs like ποικίλος (poikílos 'variegated, complex'), with a paroxytone, and Vedic Sanskrit पेशलः (peśaláḥ). The law is named after the American philologist Benjamin Ide Wheeler who published work on the topic in 1885.

## Indo-Iranian
### Brugmann's law

    In open, non-final syllables, the vowel *o is lengthened and becomes *ā. In all other contexts, *o becomes short *a. Masato Kobayashi has argued that the underlying sound change is *o to *ā, but the sound change does not occur in closed syllables to circumvent syllable-weight violations. The law is named for Karl Brugmann who first articulated the process in 1879; although Brugmann applied the term "law" to his work, the use of his name was later applied by his regular collaborator Hermann Osthoff.

### Fortunatov's law

    (Sanskrit) When Proto-Indo-European *l precedes a dental consonant, the latter becomes a retroflex consonant and the *l is deleted. Examples include जठर (jaṭhára 'belly'), which is derived from Proto-Indo-European *ǵelt-, and कुठार (kuṭāra 'ax'), derived from *kult-. The law is named after the Russian linguist Filipp Fortunatov.

### law of the palatals

    Also, Palatalgesetz. Proto-Indo-European *e palatalizes velar stops and becomes Proto-Indo-Iranian *a. The process is preceded by the delabialization of the labiovelar consonants. Although several linguists have attempted to identify the originator of the law and several have declared a supposed originator, no consensus has been reached. According to N. E. Collinge, it appears that six different linguists – Hermann Collitz, Ferdinand de Saussure, Johannes Schmidt, Esaias Tegnér Jr., Vilhelm Thomsen, and Karl Verner – discovered the law "roughly simultaneously" and "in entire independence" from one another.

## Italic
### Exon's law

    In a word with four or more syllables, if the second and third syllable are short, then the vowel of the second syllable is syncopated, though it may be restored by analogy.

### Lachmann's law

    When a short vowel is followed by an underlyingly voiced stop followed by a voiceless stop, it is lengthened. The process explains the differences between verbal forms, such as agō ('I drive') and cadō ('I fall'), and their respective derivatives, such as āctus ('made, done') and cāsus ('a fall'). Although the law was popularized by Paul Kiparsky, it is named for the German classicist Karl Lachmann who wrote about the process in 1850. It is unclear if the law applied to the whole Italic language family, but it applied at least to Latin.

### pius law

    Also, Thurneysen's law. The Proto-Italic diphthong *ūy, derived from a Proto-Indo-European sequence of *u followed by a laryngeal and *i, is fronted to *īy. Examples include Oscan piíhiúí ('pious [dative singular]') and Latin pius ('pious'), both from Proto-Italic *pīyo- derived from Proto-Indo-European *puh₂yo- 'pure'. The root is shared with the Latin pūrus ('pure'), derived from Proto-Indo-European *puH-ro- ('clean'), which did not undergo this change. Warren Cowgill argued that this law also occurred in Celtic in attempting to unify both Italic and Celtic into a double-jointed Italo-Celtic subfamily.

### Thurneysen–Havet's law

    The Proto-Italic diphthong *ow becomes *aw before a vowel. This process precedes the Proto-Italic rounding of the diphthong *ew to *ow. Examples include Latin caueō ('I am weary of'), derived from Proto-Indo-European *kowh₁-eyo-, whence Ancient Greek κοέω (koéō 'I am aware'). There appears to be some variation based on Roman social register and the process may also affect the inverted sequence *wo. For example, the Latin term vacuus exists alongside vocīuus in the work of some comedians, including in Casina, a play written by Plautus. The law is named for the Swiss linguist Rudolf Thurneysen and the French classicist Louis Havet, who appear to have developed the concept largely independently of each other in 1884 and 1885, respectively.