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.
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-.
(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).
(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.
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.
In words with a (Balto-Slavic) mobile accent paradigm, the accent was retracted from a medial onto the initial syllable.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.
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.
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.
(Lithuanian) If a word-final long vowel or diphthong is acuted, it is shortened.[clarification needed]
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.
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.
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.
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).
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 fricatives (*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.
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.
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.