Osthoff's law

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)

References

• Collinge, Neville Edgar. 1985. The laws of Indo-European. Amsterdam: John Benjamins.
• Kiparsky, Paul. 1967. A phonological rule of Greek. Glotta 44.109-34.
• Osthoff, Hermann. 1881. Review of G. Mayer, Griechische Grammatik. Philologische Rundschau 1, cols.1593ff.
• Osthoff, Hermann. 1884. Zur Geschichte des Perfects im Indogermanischen: mit besonderer Rücksicht auf Griechisch und Lateinisch. Strassburg: Trübner.
• Osthoff, Hermann. 1888. Etymologica I. Beiträge zur Geschichte der Deutschen Sprache und Literatur 13.395-463. Halle: Niemeyer.
• Ringe, Donald A. 2006. A History of English: Volume I: From Proto-Indo-European to Proto-Germanic. Oxford: Oxford University Press.