Dybo's law, or Dybo-Illič-Svityč's law, is a Common Slavic accent law named after Russian accentologists Vladimir Dybo and Vladislav Illich-Svitych.
According to the law, the accent was shifted rightward from a non-acute syllable (i.e. a long circumflex syllable, or a short syllable) to the following syllable if the word belonged to the non-mobile accentual paradigm. This produced the difference between the later accent classes A and B. The length of the previously-accented syllable remains. This is in fact the primary source of pre-tonic length in the later Slavic languages (e.g. Serbo-Croatian), because inherited Balto-Slavic vowel length had previously been shortened in pre-tonic syllables, without a change in vowel quality. (This caused the phonemicization of the previously automatic quality variations between short and long vowels — e.g. short *o vs. originally long *a.) When the newly stressed syllable was long, the accent was subsequently shifted leftward again by Ivšić's law (a.k.a. Stang's law), resulting in a neoacute accent.
- Proto-Slavic *žènāˀ 'woman' > Middle Common Slavic (MCS) *žèna (acute register in immediately post-tonic syllable shortened) > Late Common Slavic (LCS) *ženà by Dybo's law > Chakavian ženȁ
- Latin vīnum 'wine' > MCS *vîno > LCS *vīnò by Dybo's law > Serbo-Croatian víno (with long rising accent, indicating neoshtokavian accent retraction from the following syllable)
- Proto-Slavic *pîrstu 'finger' (cf. Lithuanian pir̃štas) > MCS *pь̂rstъ > *pьrstъ̀ by Dybo's law > LCS *pь̃rstъ by Ivšić's law > obsolete Russian perst, gen sg perstá
- Proto-Slavic *kàtu 'cat' > MCS *kòtъ > *kotъ̀ by Dybo's law > LCS *kõtъ by Ivšić's law > Russian kot, gen sg kotá
- Proto-Slavic *vàljāˀ 'will' > MCS *vòlja > *vòljā by Van Wijk's law (i.e. vowel lengthening after /j/) > *voljâ by Dybo's law > LCS *võlja by Stang's law > Russian vólja, Slovak vôľa (note, Slovak ô [u̯o] specifically reflects neoacute)