Thanks to a comment from Aldiboronti in this Wordorigins thread, I learned an interesting fact: the word truce is essentially just the plural of true. As the OED puts it:
[ME. trewe and triewe, mostly in pl. form trewes and triewes:—OE. tréow n. masc. (fem. pl. tréwa), ‘truth or fidelity to a promise, good faith, assurance of faith or truth, promise, engagement, covenant, league’, = OEFris. tríuwe, OWFris. and MDu. trouwe (Du. trouw), OS. treuwa, tríuwa, OHG. tríuwa (MHG. triuwe, Ger. treue):—WGer. *trewwa, Goth. triggwa ‘covenant’ (whence late L. and Romanic tregua, treuga, F. trève); also, in ablaut form, OE. trúwa n. masc. and pl. -an; = ON. trúa, trú, Norw. trū, Sw. trōa: see TRUE a. Already in OE. the pl. tréwa was often used in the sense of the sing.; this became still more frequent with the ME. pl. trewes, triues, triwes, trues, and finally this, as trews, trewse, truse, truce, became the received sing. (app. in reference to the pledges or engagements given by both parties), with a new pl. truses, truces, when required. Cf. cherries, pease. See also trève from French, and the rare treuges after MLat. treugas.]
But what I want to know is: if it’s originally a plural, why doesn’t it have a voiced final consonant? In other words, why isn’t it trewes or trues?