Glossary of sound laws in the Indo-European languages

This glossary gives a general overview of the various sound laws that have been formulated by linguists for the various Indo-European languages. A concise description is given for each rule; more details are given in their articles.

Within Proto-Indo-European or multiple branches
boukólos rule: Labiovelars lose their labialization and become plain velars when preceded or followed by *w or *u. This law remained productive into Proto-Germanic, where it also came to apply to labiovelars preceded by -um- or -un-. Grassmann's law: (Greek, Indo-Iranian) When an aspirated consonant is followed by another aspirated consonant in the next syllable, the first loses its aspiration. The law was or remained productive after the Greek devoicing of aspirates, and the change of *s to *h (which counted as an aspirate). Kortlandt effect: In a word of three syllables with a vowel pattern é-o-X (X is any vowel), the accent is advanced to the middle syllable, giving e-ó-X. Osthoff's law: (All but Indo-Iranian and Tocharian) When a long vowel is followed by a sonorant and another consonant in the same syllable, it is shortened. Pinault's law: Laryngeals are dropped before consonantal *y. Ruki sound law: (Balto-Slavic, Albanian, Armenian, Indo-Iranian) *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. Siebs' 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: Word-finally, when *s or *h₂ is preceded by a sonorant preceded by a vowel, it is dropped and the vowel is lengthened. If the resulting sequence is *-ōn, the *n is also dropped, giving *-ō. Weise's law: Palatovelar consonants *ḱ *ǵ *ǵʰ are depalatalized when preceding *r.
 * d may be debuccalized into the laryngeal *h₁ before consonants.
 * kʷetwóres rule:

Balto-Slavic
Hirt's law: If a syllable ends in a vowel (not a diphthong) followed by a laryngeal, then the accent is retracted onto that syllable if it originally fell on the following syllable. The law applies to the original PIE accent placement, but after levelling 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 (*a, *e, *i, *o, *u) are lengthened before unaspirated voiced stops (*b, *d, *g, but not *ǵ). The newly lengthened vowel receives the "acute register". The law may only have applied to vowels that were followed by a stop in the same syllable.

Baltic
De Saussure's law: (Lithuanian) 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: When a vowel receives an accent, it takes on the intonation of the syllable to its right (i.e., the following syllable). Leskien's law: (Lithuanian) If a word-final long vowel or diphthong is acuted, it is shortened. This law precedes Hjemslev's law.

Slavic
Dybo'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. Havlík's law: Yers (the vowels *ь and *ъ) became "strong" or "weak" in an alternating pattern, depending on their position in a word. A yer was weak unless the next syllable contained a weak yer, then it became strong. Weak yers were eventually lost, while strong yers were lowered and became other vowels. Ivšić's law: Accented weak yers (according to Havlík's law) lost their accent to the preceding syllable, which received a "neoacute" accent. Meillet's law: In words with a mobile accent paradigm, if the first syllable is accented with a rising (acute) accent, it is converted into a falling (circumflex) accent. Shakhmatov's law: (No longer widely accepted) Slavic short falling (circumflex) accents shift leftward (i.e., to the preceding syllable). 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.

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

Germanic
Cowgill's law: (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: 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: The three series of Indo-European plosives undergo a chain shift. Voiceless plosives become fricatives (*p, *t, *k, *kʷ > *f, *þ, *h, *hʷ), plain voiced plosives are devoiced (*b, *d, *g, *gʷ > *p, *t, *k, *kʷ), aspirated plosives become voiced plosives (*bʰ, *dʰ, *gʰ, *gʷʰ > *b, *d, *g, *gʷ). In a sequence of two voiceless obstruents (following any voicing assimilation), only the first becomes a fricative, the second remains a plosive. Holtzmann's law: (North and East Germanic) The geminated glides *jj and *ww are hardened into geminate plosives. *jj becomes *ggj in North Germanic and *ddj in East Germanic, *ww becomes *ggw in both. Ingvaeonic nasal spirant law: (Ingvaeonic languages) When followed by a fricative, is lost and the preceding vowel is lengthened and nasalised. Kluge's law: (Not generally accepted) PIE plosives merge with a following *n and become geminate voiceless plosives in Germanic. Sievers' 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. 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.

Indo-Iranian
Bartholomae's law: If cluster of two or more obstruents contains at least one voiced aspirated consonant, the whole cluster becomes voiced and aspirated. Brugmann's law: When it is an ablaut alternant of *e, the vowel *o is lengthened and (after merging) becomes *ā when it stands at the end of a syllable.

In all words or word-groups of four or more syllables bearing the chief accent on a long syllable, a short unaccented medial vowel was necessarily syncopated, but might be restored by analogy

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. Lachmann's law: When a short vowel is followed by an underlyingly voiced stop followed by a voiceless stop, it is lengthened.