Ruki sound law
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Ruki (or iurk) refers to a sound change in Balto-Slavic, Albanian, Armenian, and Indo-Iranian, wherein an original /s/ phoneme changed into [ʃ] after the consonants /r/, /k/, and the semi-vowels /u̯/, /i̯/, or:
- s > ʃ / r, u, K, i _
Specifically, the initial stage involves the coronal sibilant [s] retracting to an apical [s̺] after *i̯, *u̯, *r, or a plain velar consonant (that is, *k or *g) having developed from earlier *k, *g, *gʰ. In the second stage, [s̺] further retracted to a voiceless postalveolar fricative [ʃ]; this stage was reached universally. The third stage involves retroflexion (cf. Sanskrit ष [ʂ] and Proto-Slavic) and is due to levelling of the sibilant system. Similarly, levelling brought about the fourth and final phase involving the retraction to velar [x] in Slavic and some Middle Indian languages (with parallels in languages like Spanish). This rule was first formulated for the Indo-European languages by Holger Pedersen, and it is known sometimes as the Pedersen law.
The name "ruki" comes from the sounds (r, u, k, i) which cause the phonetic change.
Applications to language groups 
The rule was originally formulated for Sanskrit. It was later proposed to be valid in some degree for all Satem languages, and exceptionless for Indo-Iranian languages. In Baltic and Albanian, it is more or less limited or affected by other sound laws. Nevertheless, it has to have been universal in these branches of the IE languages, and the lack of Slavic reflexes before consonants is due rather to their merger with the reflexes of other sibilants.
Exceptions in Slavic languages 
In Slavic languages the process is regular before a vowel, but it does not take place before consonants. The final result is the voiceless velar fricative *x, which is even more retracted than the *š. This velar fricative changed back into *š before a front vowel or the palatal approximant *j.
Exceptions in Indo-Iranian languages 
In Indo-Iranian *r and *l merged, and the change worked even after the new sound. This has been cited as evidence by many scholars as an argument for the later influence of Iranian languages on Proto-Slavic. There are obvious drawbacks in the theory. First, the two sounds must have been very close (r/l), so that both could have triggered the change in Indo-Iranian. Second, there are no real examples of this change working in Slavic, and it is also doubtful that only this change (ruki) and no other such change of sibilants (e.g. s → h) was borrowed into Slavic.
Further reading 
- Charles Prescott. "Germanic and the Ruki Dialects"
- Mayer, Harvey E. (1980), "Baltic Membership in the West Satem Group", Journal of Baltic Studies 11 (4): 356–366