Introduction -- The rise of Slavic x in the non-initial position the ruki rule — -- Conditions and area of retraction -- The second step retraction s > x -- Phonologization -- Relative and absolute chronology and the question of Iranian influence -- The survey of opinions on the rise of initial Slavic x -- Potential sources of the word-initial x -- Slavic x of the onomatopoeic origin -- PS loanwords with initial x -- Slavic x as a continuation of IE *x or *kh -- Slavic x IE *sk-, *k -- Slavic x IE *sg-, *g -- Slavic x— IE *s -- Slavic x IE *k’ -- Slavic x as a prothetic sound -- Résumé of potential sources of Slavic x -- Etymological analysis of PS lexicon with the initial x-/š -- The interpretation of the etymological analysis -- Findings -- Interpretation -- Conclusion -- Bibliography