This article deals with Common Slavic rows of lexical oppositions with x ~ sk ~ šč, going back to primordial Indo-European sound clus- ters *sk or *ks, and presents an attempt at establishing laws and rules of their occurrence. There is one more Indo-European language, namely Albanian, in which there was a similar regular transition of etymologi- cal *sk, *ks into *h in initial, intervocal and final positions in the words. All examples of similar opposition known nowadays from ESSJA and other accessible publications are presented for further consideration of the similar phenomena. About 160 examples of different degrees of re- liability are submitted. The author deduces some rules on the basis of comparisons: 1) etymological initial *sk- regularly passed into *x- before back vowels. Reconstruction of forms with s mobile is an anachronism. 2) etymological initial *sk- has regularly changed into *šč- before front vowels. 3) Etymological initial *sk- was kept only after a prefix *ob-. 4) It is possible to explain preservation of etymological initial *sk- in some cases by a loss of a prefixal morpheme: *skъrbь from *obskъrbь. The mechanism of mutation seems to be as follows: sk- has changed into ks- (this stage of development: Sanskrit kṣoda and Greek ), and then by the notorious rule of RUKI ks- has changed into kh- and later into x-. The etymological confluence sk- before a back vowel is kept only after prefix *ob- (*obskalъ, *obskǫděti, *obskomina). Regular opposition initial x-: initial šč-, can never be sk-: šč-, therefore ščel’, ščyolka correlate with oskal but skala, skol < *sъkala, *sъkolъ. In this case we have an etymo- logical opposition *xo-: *sъ-ko-.