Exon's Law: A vowel-reduction rule in Proto-Italic, precursor to languages like Latin, Oscan, and Umbrian.

Basically, in words with four syllables whose two middle syllables are short, the word's second vowel is lost.
(In the examples, the lost vowel is put in bold.)

*deksiteros > Latin dexter "right" (with loss of the *i; the -os is lost later)
*kwinkwedekem > Latin quīndecim "15"
*hostipotis > Latin hospes "guest".

If a word's paradigm included tetrasyllables, shorter forms acquired the change by analogy, thus *prīsmokaps > L prīnceps "prince", because of forms like the accusative prīncipem from *prīsmokapem.


  • When the consonant between the two medial syllables is l or r, the loss generally occurs in the third syllable, thus ūsurpō from *ussurupō.
  • Anaptyctic vowels arose afterwards, and are exempt from the rule, so nōbilitās remains, whose -bil- is from an original *-θl-.