(obj-d)Winter's law (WL) specifies the prehistoric glottalisation, traditionally known as acuting, of a vowel in Balto-Slavic (BSl.) when followed by a Proto-Indo-European (PIE) media, either immediately or with intervening resonant (Winter 1978; Kortlandt 1978b etc. etc.; Young 1990, 1991). The PIE mediae are now recognised by many scholars to have been preglottalised stops despite continued symbolisation as plain voiced stops (b d g etc.). The same acuting can also be the reflex of any of the three PIE consonants called laryngeals.

The principal languages in which acute syllables have a contour of intensity phonemically opposed to a non-acute (“circumflex” contour or simple brevity) fall into two groups. In Lithuanian and SerboCroat, the glottalisation tended to shut down the intensity of the latter part of the vowel or diphthong resulting in a contour of falling intensity. In philological works this is marked in Lithuanian by an acute (´) over vowels and either an acute or a grave (`) over the first component of diphthongs, including those whose second component is m, n, r or l; and in SerboCroat by a double grave (``). In Latvian, Old Prussian and Slovene, on the other hand, a glottal squeeze led to an enhancement of the latter part of the vowel's or diphthong's acoustic intensity resulting in a rising or level contour marked in Latvian by a tilde (sustained tone) on syllables already accented by Hirt's law or earlier and by the French circumflex (broken tone) on other syllables; in the Old Prussian Enchiridion by a macron over vowels and the second component of diphthongs ending in i or u; and in Slovene by an acute (´).

Non-acute is indicated in Lithuanian by over long vowels and the second component of diphthongs, and by (`) over short vowels; in Latvian and Old Prussian by (`) and, respectively, over the first component of diphthongs and by nothing over vowels; in SerboCroat and Slovene sometimes by, but the situation in the Slavic languages is generally more complicated than in the Baltic. (On the Old Prussian markings see, e.g. Schmalstieg 1974: 22–24; Endzelin 1944: 28–31).