“…Impressed by the power of Charlemagne (died
814), Slavic people took his name Karl as the common name for ‘the monarch, sovereign.’ However, according to
the rules of late Proto-Slavic phonology, the consonant cluster [rl] could not
hold. The pressure to dispose of it was common to all speakers of Slavic
dialects, yet the way the problem was resolved turned out to be different in
the South, North-East, and North-West. The Southern Slavs employed a simple
metathesis ar > ra: Serbian кра̑љ, Bulgarian крал (also Czech král); in Polish, the metathesis was accompanied by a
shortening and eventual change of the vowel: król; in East Slavic languages,
the problem was resolved by adding another vowel between the consonants (the
so-called pleophony): Russian король. The meaning remained the same everywhere: ‘king.’”
[from the introduction of Boris Gasparov’s Old Church Slavonic]