The theory accounts for a number of otherwise puzzling facts. Sanskrit has pitaraḥ, mātaraḥ, bhrātaraḥ for "fathers, mothers, brothers" but svasāraḥ for "sisters", a fact neatly explained by the traditional reconstruction of the stems as for "father, mother, brother" but for "sister". Similarly, the vast majority of n-stem nouns in Indic have a long stem-vowel, such as brahmāṇaḥ "Brahmins", śvānaḥ "dogs", correlating with information from other Indo-European languages that they were originally *on-stems. There are also some exceptions, including ukṣan- "ox", which in the earliest Indic text, the Rigveda, shows forms as ukṣǎṇaḥ "oxen". They were later replaced by "regular" formations, but the notion that the short stem vowel might have been from an -stem is supported by the unique morphology of the Germanic forms: Old Englishoxa nominative singular "ox", exen plural; the Old English plural stem, such as the nominative, continues Proto-Germanic *uhsiniz < *uhsenez, with e > i in noninitial syllables followed, in Old English, by an umlaut. This is the only Old English n-stem that certainly points to -vocalism, rather than -vocalism.
Exceptions
The rule seems to apply to only an that is the ablaut alternant of. Non-apophonic, with no alternant, developed into Indo-Iranian : "master, lord" > Sanskrit pati-, not ˣpāti. Alternatively, it is explained by the voiceless consonant after the vowel, but to adopt a form of the sound law that affects only in open syllables, followed by a voiced consonant, seems to be a slim basis for a rule that is so general in Indo-Iranian. Limiting the original environment to that before voiceless consonants then requires levelling of long-vowel forms to perfects and nouns with final voiceless consonants in Pre-Indo-Iranian. That faces particular problems in explaining the archaic form ānāśa 'he/she has reached' <, with its very idiosyncratic synchronic relation to Sanskrit √aś 'reach'. Several exceptions can be addressed by the laryngeal theory. The form that is traditionally reconstructed as *owis "sheep", is a good candidate for again reconstructing, as , rather than an ablauting o-grade. Perhaps the most convincing confirmation comes from the inflection of the perfect: a Sanskrit root like sad- "sit" has sasada for "I sat" and sasāda for "he, she, it sat". The conventional 19th century wisdom saw it as some kind of "therapeutic" reaction to the Indo-Iranian merger of the endings *-a "I" and *-e "he/she/it" as -a, but it was troubling that the distinction was found only in roots that ended with a single consonant. That is, dadarśa "saw" is both first- and third-person singular, but a form like ˣdadārśa would have been allowed by Sanskrit syllable structure. The mystery was solved when the ending of the perfect in the first person singular was reanalyzed, on the basis of Hittite evidence as *-h₂e, beginning with an a-colouring laryngeal. In other words, while Brugmann's Law was still operative, a form of the type in the first-person singular did not have an open root syllable. A problem for the interpretation is that roots that quite plainly must have ended in a consonant cluster including a laryngeal, such as jan- < "beget" and therefore should have had a short vowel throughout nevertheless show the same patterning as sad-: jajana first-person singular, jajāna third-person singular. Whether that is a catastrophic failure of the theory or just levelling is unsure, but after all, those who think the pattern seen in roots like sad- has a morphological, not phonological, origin, have their own headaches, such as the total failure of this "morphological" development to include roots ending in two consonants. Such an argument would anyway cut the ground out from under the neat distributions seen in the kinship terms, the special behaviour of "ox" and so on. Perhaps the most worrisome data are adverbs such as Sanskrit prati, Greekpros and some other forms, all of which appear to have ablauting vowels. They also all have a voiceless stop after the vowel, which may or may not be significant.
Current status
For all of its charms, Brugmann's Law now has few supporters, and even Brugmann himself eventually gave up on it). Jerzy Kuryłowicz, the author of the explanation of the sasada/sasāda matter , eventually abandoned his analysis for of an appeal to the theory of marked vs unmarked morphological categories. Scholars who still accept Brugmann's Law include Martin Joachim Kümmel, who compares it to developments in Anatolian and Tocharian languages and to Saussure's losses of laryngeals near *o in the internal reconstruction of pre-PIE *o as longer than *e''.
