There is a well-known rule of Russianwhereby /i/ is said to be realized as  after non-palatalized consonants. Somewhat less well known is another allophonic rule of Russian whereby only [i], and not , can follow velars within a morphological word. This latter rule came about due to a sound change in East Slavic called post-velar fronting here: k > kji(and similarlyfor the other velars). This paper examines this sound change in depth, and argues that it can be adequately explained only by appeal to the functional notions of perceptual distinctiveness of contrast and neutralization avoidance. Further, these notions crucially require a systemic approach to phonology, in which the wellformedness of any form must be evaluated with reference to the larger system of contrasts it enters into. These notions are formalized in a modified version of Dispersion Theory (Flemming 1995a), a systemic theory that incorporates these functional notions into Optimality Theory (Prince and Smolensky 1993).