During the Late Common Slavic period, the short vowels *ь and *ъ (known as yers, also written *ĭ *ŭ) developed into "strong" and "weak" variants according to Havlík's law.
In conservative Serbo-Croatian dialects of Čakavian and Old Štokavian (e.g., Slavonian), this neoacute is preserved as a separate tone, distinct from the old acute and circumflex.
*pirstu̍ > Common Slavic *pьrstъ̍ > *pь̃rstъ (Čakavian pr̃st, Russian perst, N pl perstý) Retraction also occurred on medial long circumflexed (i.e., non-acuted) syllables; for example, on verbs in *-iti.
[2] This retraction is uncontroversial if the preceding syllable is long; in the case of short preceding syllables, it is generally accepted,[3] but some[4] argue that it is analogical to the long neoacute in individual (mostly West Slavic) languages.
Additionally, Ivšić's law explains the acute accent on certain jā-stem nouns such as sũša (Slavonian Štokavian dialects), vȍlja (with shortened neoacute).