Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
sheath (n., v.), sheathe (v.)
The noun plural is either sheathes (with a voiced -thz sound for the final consonant cluster, to rhyme with breathes), or sheaths (with the final cluster a voiceless -ths, to rhyme with Keith’s). The third person singular present tense of the verb has the same two pronunciations and spellings as the noun plural. The past tense and past participle of the verb are both spelled sheathed but pronounced with either a voiced (-thd) or voiceless (-tht).
