Proto-Indo-Iranian historical phonology

The most distinctive phonological change separating Proto-Indo-Iranian from Proto-Indo-European is the collapse of the ablauting vowels *e, *o, *a into a single vowel, Proto-Indo-Iranian *a (but see Brugmann's law). Grassmann's law, Bartholomae's law, and the Ruki sound law were also complete in Proto-Indo-Iranian.

A fuller list of some of the hypothesized sound changes from Proto-Indo-European to Proto-Indo-Iranian follows:


 * The Satem shift, consisting of two sets of related changes. The PIE palatals *k̂ *ĝ *ĝʰ are fronted or affricated, eventually resulting in PII *ĉ, *ĵ, *ĵʰ, while the PIE labiovelars *kʷ *gʷ *gʷʰ merge with the velars *k *g *gʰ.
 * The PIE syllabic liquids *l̥, *r̥ merge as *r̥.


 * The PIE syllabic nasals *m̥ *n̥ merge with *a.


 * Bartholomae's law: an aspirate immediately followed by a voiceless consonant becomes voiced stop + voiced aspirate. In addition, dʰ + t > dzdʰ.
 * The Ruki rule: *s is retracted to *š when immediately following *r *r̥ *u *k or *i. Its allophone *z likewise becomes *ž.
 * Before a dental occlusive, *ĉ becomes *š and *ĵ becomes *ž. *ĵʰ also becomes *ž, with aspiration of the occlusive.


 * The sequence *ĉs was simplified to *šš.
 * The "second palatalization" or "law of palatals": *k *g *gʰ develop palatal allophones *č *ǰ *ǰʰ before the front vowels *i, *e.


 * Brugmann's law: *o in an open syllable lengthens to *ō.
 * The vowels *e *o merge with *a. Similarly, *ē, *ō merge with *ā.  This has the effect of giving full phonemic status to the second palatal series *č *ǰ *ǰʰ.


 * In certain positions, laryngeals were vocalized to *i. This preceded the second palatalization.
 * Following a consonant, and preceding a consonant cluster
 * Following a consonant and word-final


 * The Indo-European laryngeals all merged into one phoneme *H, which may have been a glottal stop. This was probably contemporary with the merging of *e and *o with *a.
 * According to Lubotsky's Law, *H disappeared when followed by a voiced nonaspirated stop and another consonant: