Osthoff's law

Definition
Osthoff's law is a tendency of Proto-Indo-European long vowels to shorten when they were followed by a sonorant and another consonant. The law was proclaimed in 1881 and again in 1884 by the German linguist Hermann Osthoff (1847-1909). It is apparent mostly in Greek, but there were attempts to widen the domain of its application to Latin, Gothic, Baltic and even Germanic, though the evidence is weak (Collinge 1985: 127, Ringe 2006: 1975-77). The law was formulated as follows:
 * Jeder lange vocal ist in der stellung vor sonorlaut [...] und einem weiteren consonant innerhalb desselben wortes urgriechisch verkürzt worden. (Osthoff 1884: 84-85 as cited in Collinge 1985: 127)