Hirt's law, named after Hermann Hirt, who originally postulated it in 1895, is a Balto-Slavic sound law that states in its modern form that the inherited Proto-Indo-European stress would retract to a non-ablauting pretonic vowel or a syllabic sonorant if it was followed by a consonantal (non-syllabic) laryngeal that closed the preceding syllable.
Hirt's law did not operate if the laryngeal preceded a vowel, or if the laryngeal followed the second component of a diphthong. Therefore, Hirt's law must be older than then the loss of laryngeals in prevocalic position (in glottalic theory formulation: to the merger of glottalic feature of PIE voiced stops who dissolved into laryngeal and buccal part with the reflexes of the original PIE laryngeals), because the stress was not retracted in e.g. *tenh₂wós (Ancient Greek tanaós, Sanskrit tanú) "thin" > Latvian tiêvs[clarification needed], and also older than the loss of syllabic sonorants in Balto-Slavic, as can be seen from the abovementioned reflexes of PIE *pl̥h₁nós, and also in e.g. PIE *dl̥h₁gʰós "long" (compare Sanskrit dīrghá, Ancient Greek dolikhós) > Lithuanian ìlgas, Latvian il̃gs, Croatian/Serbian dȕg.
It follows from the above that Hirt's law must have preceded Winter's law, but was necessarily posterior to Balto-Slavic oxytonesis (shift of stress from inner syllable to the end of the word in accent paradigms with end-stressed forms), because oxytonesis-originating accent was preserved in non-laryngeal declension paradigms; e.g. the retraction occurs in mobile *eh₂-stems so thus have dative plural of Slovene goràm and Chakavian goràmi (< PBSl. *-eh₂mús), locative plural of Slovene and Chakavian goràh (< PBSl. *-eh₂sú), but in thematic (o-stem) paradigm dative plural of Slovene možȇm (< PBSl. *-mús), locative plural of Slovene možéh and Chakavian vlāsíh (< PBSl. *-oysú). The retraction of accent from the ending to the vowel immediately preceding the stem-ending laryngeal (as in PBSl. reflex of PIE *gʷrH-) is obvious. There is also a strong evidence that the same was valid for Old Prussian (in East Baltic dative and locative plural accents were generalized in non-laryngeal inflections).
From the Proto-Indo-European perspective, the importance of Hirt's law lies in the strong correspondence it provides between the Balto-Slavic and Vedic/Ancient Greek accentuation (which more or less intactly reflects the original PIE state), and somewhat less importantly, provides a reliable criterion to distinguish the original sequence of *eH from lengthened grade *ē, as it unambiguously points to the presence of a laryngeal in the stem.