Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
PRONUNCIATIONS OF WORDS ENDING IN -TH, -THE
Three generalizations apply to most words ending in -th and -the when they undergo functional shift to become verbs: they add a final -e in spelling, a short medial vowel becomes long, and the voiceless th, as in thigh, becomes voiced, as in thy: breath (BRETH) become breathe (BREETH); sheath (SHEETH) becomes sheathe (SHEETH); teeth (TEETH) becomes theethe (TEETH)
