Proto-Balto-Slavic language different from Balto-Slavic
Accentual mobility
Both Proto-Indo-European and Proto-Balto-Slavic had systems of accentual mobility, in which the accent would be placed on different syllables in different inflections of the same word. The systems of the two languages are vastly different in detail, however. PIE mobility and Balto-Slavic mobility are unconnected; Balto-Slavic mobility is not inherited from Proto-Indo-European, but formed entirely anew. PIE mobility was entirely lost in the early stages of Proto-Balto-Slavic, by eliminating the accentual distinction between "strong" and "weak" forms (accented further to the left and right, respectively), usually in favour of the weak forms, i.e. accent on the suffix syllable for proterokinetic stems. Hysterokinetic stems already effectively had fixed accent on the first syllable after the root, and retained this. Amphikinetic stems joined this pattern.
The exact process by which the new Balto-Slavic type of mobility arose is still a hotly debated topic, although some details are clear. As a general rule, nouns that had the accent on the ending ("oxytones") became mobile, while those that had accent on the root ("barytones") retained their fixed accent. Words with mobile accent had accent on the leftmost syllable of the stem in some forms, accent on the rightmost syllable of the ending in others (modified by Hirt's law). A special case is formed, however, by oxytone neuter o-stems, which under some conditions appear as AP b in Slavic instead of the expected AP c.
The following sections lay out the explanations for mobility given by various linguists. The explanations are very different in nature, and sometimes also give different results for different cases. The "incorrect" outcomes are in turn explained by each in their own way.