In comparative Germanic grammar it is a commonplace assumption that IE ē first became æ in Proto-Germanic and then developed into ā in the parent forms of both North and West Germanic. Prokosch, for example, remarks, Two changes [affecting IE long vowels] are generally ascribed to Primitive Germanic: that of ā to ō and of ē to ǣ. The latter is largely a “Phonetic Interpolation”, i.e., the assumption of a transitional stage between ē and ā. In Gothic, ǣ reverts to ē by a different process of tensing (raising and fronting); in North and West Germanic, ā results; this develops secondarily into ǣ in Old English, and ē in Frisian.'