Under Hirt's law, a non-initial accent was retracted to a non-ablauting vowel, if it was followed by a consonantal (non-syllabic) laryngeal that closed the preceding syllable. The retraction did not take place if the laryngeal did not immediately follow the syllable nucleus, i.e. if the nucleus contained a vocalic or sonorant diphthong such as ey or en. However, the retraction did take place onto a syllabic sonorant, showing that Hirt's law applied before the epenthesis of the high vowels *i and *u before syllabic sonorants.
Hirt's law followed the creation of the distinction between fixed and mobile accentual paradigms in early Balto-Slavic. In two-syllable forms (root plus vocalic ending), only mobile paradigms had forms with non-initial stress, so Hirt's law could only operate on them. In longer stems, both fixed and mobile paradigms were affected.
In fixed paradigms
Hirt's law only affected fixed-accent forms if they had three or more syllables, since fixed-accent paradigms could not have forms with a final accent, only mobile paradigms could. The result was a straightforward shifting of the columnar accent one syllable to the left, with the paradigm remaining fixed.
In mobile paradigms
In mobile paradigms, Hirt's law affected the final-accented forms. This is visible in disyllabic endings of the ā-stem nouns, which have penultimate accent where the equivalent endings of other inflection classes have final accent. Late Common Slavic reflects this as a distinction of neoacute (from an original final accent, by Ivšić's law) versus normal acute (from an original penultimate accent).
In the locative plural:
- In the ā-stems: Pre-Balto-Slavic *-áHsu > (by oxytonesis) *-aHsú > (by Hirt's law) *-áHsu > Proto-Balto-Slavic *-ā́ˀsu > Late Common Slavic *-àxъ > Slovene and Chakavian -àh, with a short vowel resulting from a Slavic acute.
- In the o-stems: Pre-Balto-Slavic *-óyšu > (by oxytonesis) *-oyšú > Proto-Balto-Slavic *-aišú > Middle Common Slavic *-ěxъ́ > (by Ivšić's law) Late Common Slavic *-ě̃xъ > Slovene -éh, Chakavian -íh, both with a long vowel resulting from a Slavic neoacute.
In cases of two-syllable forms (such as the nominative singular of many nouns) where the accent was retracted from the second onto the first syllable by Hirt's law, such forms would have stood alongside the older, existing initial-accented forms in a mobile paradigm. This disrupted the normal accentuation of mobile words, and was resolved in one of two ways:
- Convert the paradigm to a fixed-accent paradigm, by shifting all remaining non-initial accents onto the first syllable.
- Pre-Balto-Slavic *duHmós "smoke" > (by Hirt's law, then conversion to fixed accent) *dúHmos > Proto-Balto-Slavic *dū́ˀmas > Lithuanian dū́mai (AP 1), Late Common Slavic *dỳmъ (AP a)
- Pre-Balto-Slavic *griHwáH "?" > (by Hirt's law, then conversion to fixed accent) *gríHwaH > Proto-Balto-Slavic *grī́ˀwāˀ > Latvian grĩva, Late Common Slavic *grìva (AP a)
- Analogically restore the normal mobile accentuation, by moving the accent back from the first onto the second syllable and thus undoing Hirt's law.
- Pre-Balto-Slavic *suHnús "son" > (by Hirt's law) *súHnus > (analogical restoration) *suHnús > Proto-Balto-Slavic *sūˀnús > Lithuanian sūnùs (AP 3), Late Common Slavic *sy̑nъ (AP c)
- Pre-Balto-Slavic *giHwós "alive" > (by Hirt's law) *gíHwos > (analogical restoration) *giHwós > Proto-Balto-Slavic *gīˀwás > Lithuanian gývas (AP 3), Late Common Slavic *žȋvъ (AP c)
Here, Lithuanian accent paradigm 1 and Slavic accent paradigm a reflect a Balto-Slavic fixed-accent paradigm, while Lithuanian AP 3 and Slavic AP c reflect a Balto-Slavic mobile paradigm. The Latvian sustained tone ˜ is an indirect reflex of a fixed-accent paradigm, contrasting with the broken tone ˆ which would reflect a mobile paradigm. Among the forms that remained mobile, only Lithuanian sūnùs preserves the inherited final accent. In Slavic, the paradigms preserve the mobile accent, but the original nominative singular forms were replaced by the accusative singular, which was identical other than the accent.
- Hermann Hirt, Der indogermanische Akzent: Ein Handbuch, Strassburg, 1895, p. 94
- Slavic Accentuation - A Study in Relative Chronology, Frederik Kortlandt, 1975