Generally speaking, Saussure’s conceptions and the theses of the Prague Linguistic Circle are considered to be two variants of the same phonological theory. From a review of the changes brought about in linguistic thinking by the Mémoire sur le système primitif des voyelles dans les langues indo-européennes (and by Saussure’s later works), we here compare Saussurian Principles with Trubetskoy’s Grundzüge der Phonologie and, Jakobson’s reading of them after 1945 in his rivalry with American structuralists. We try to give a detailed explanation of the differences between the schools of phonology in reference to the conditions of their emergence and to their objectives. We pay particular attention to the place their work gives to the phonetics.