The ruki sound law, also known as the ruki rule or iurk rule, is a historical sound change that took place in the satem branches of the Indo-European language family, namely in Balto-Slavic, Albanian, Armenian, and Indo-Iranian. According to this sound law, an original *s changed to *š (a sound similar to English "sh") after the consonants *r, *k, *g, and the semi-vowels *w (*u̯) and *y (*i̯):
Specifically, the initial stage involves the retraction of the coronal sibilant *s after semi-vowels, *r, or a velar consonant *k or *g (developed from earlier *k, *g, *gʰ). In the second stage, leveling of the sibilant system resulted in retroflexion (cf. Sanskrit ष [ʂ] and Proto-Slavic), and later retraction to velar *x in Slavic and some Middle Indian languages. This rule was first formulated by Holger Pedersen, and it is sometimes known as Pedersen's law, although this term is also applied to another sound law concerning stress in the Balto-Slavic languages.
The name "ruki" comes from the sounds (r, u̯, K, i̯) which triggered the sound change. The law is stated as a mnemonic rule because the word ruki means hands in many Slavic languages.
The rule was originally formulated for Sanskrit. It was later proposed to be valid in some degree for all satem languages, and exceptionless for the Indo-Iranian languages. (There appears to be one exception at least in some Nuristani languages, however.) In Baltic and Albanian, it is limited or affected to a greater or lesser extent 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.
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 *y.
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.