Osthoff's law

Osthoff's law is an Indo-European sound law which states that long vowels shorten when followed by a resonant (Proto-Indo-European language (PIE) *m, *n, *l, *r, *y, *w), followed in turn by another consonant (i.e. in a closed syllable environment). It is named after German Indo-Europeanist Hermann Osthoff, who first formulated it.

The law operated in most of the Proto-Indo-European daughter languages, with notable exceptions being the Indo-Iranian and Tocharian branches in which the difference between long and short PIE diphthongs was clearly preserved.

Compare:

• PIE *dyēws "skyling, sky god" > Vedic Sanskrit dyā́us, but Ancient Greek Ζεύς, with ordinary diphthong.
• PIE *bʰerHǵeh₂ "birch" > PBSl. *bḗrźās (laryngeal regularly dropped and root vowel lengthened) > *béržās > Lithuanian béržas, Serbo-Croatian brȅza (by liquid metathesis).

The term Osthoff's law is usually properly applied to the described phenomenon in Ancient Greek, which itself was an independent innovation from similar developments occurring in Latin and other Indo-European languages. However, often it is used in a loose sense, as a cover term referring to all shortening of long diphthongs in closed syllables.

Osthoff's law was in some version valid for Greek, Latin, Celtic and Balto-Slavic, but not for Indo-Iranian and Tocharian. It also probably applied in Germanic, although there is very little evidence to support or refute that claim.