Variable Finals in Proto-Sino-Tibetan

In many Tibeto-Burman languages we find that there are a number of forms that are clearly related though differ in one segment. In some cases these variations may be due to regular or common alternations, such as in Tibetan, where you have dental suffixes that can nominalize a verb (e.g. rkun-po 'thief', from rku 'steal') . In other cases we cannot find any morphological reason for the variation, even though the variation may involve the same segments, as in Tibetan bka, skad 'speech'. When we reconstruct the Proto-Tibeto-Burman provenience of these cognates, we sometimes have no way of knowing which form is older, so we must reconstruct two forms that are clearly related, that are what James A. Matisoff has dubbed 'allofams'. On the Chinese side of Sino-Tibetan we find similar alternations among cognate forms, as in 亡 *mjaŋ, 無 *mjag 'negative/not have'; 往 *gwjaŋ, 于 *gwjag 'go'.