The question then is, which of these various sounds of the digamma is represented by the Latin u consonant, or does it represent all, or none, of these.
Speaking of f, Priscian says:
[Keil. v. II. p. 35.] Antiqui Romanorum Aeolis sequentes loco aspirationis eam (f) ponebant, effugientes ipsi quoque aspirationem, et maxime cum consonante recusabant eam proferre in Latino sermone. Habebat autem haec f littera hunc sonum quem nunc habet u loco consonantis posita, unde antiqui af pro ab scribere solebant; sed quia non potest vau, id est digamma, in fine syllabae inveniri, ideo mutata in b. Sifilum quoque pro sibilum teste Nonio Marcello de Doctorum Indagine dicebant.
And again:
[Prisc. Keil. v. II. p. 15.] In b etiam solet apud Aeolis transire ϝ digamma quotiens ab ρ incipit dictio quae solet aspirari, ut ῥήτωρ, βρήτωρ dicunt, quod digamma nisi vocali praeponi et in principio syllabae non potest. Ideo autem locum transmutavit, quia b vel digamma post ρ in eadem syllaba pronuntiari non potest. Apud nos quoque est invenire quod pro u consonante b ponitur, ut caelebs, caelestium vitam ducens, per b scribitur, quod u consonans ante consonantem poni non potest. Sed etiam Bruges et Belena antiquissimi dicebant, teste Quintiliano, qui hoc ostendit in primo institutionum oratoriarum: nec mirum, cum b quoque in u euphoniae causa converti invenimus; ut aufero.
[Quint. I. v. 69.] Frequenter autem praepositiones quoque copulatio ista corrumpit; inde abstulit, aufugit, amisit, cum praepositio sit ab sola.
It is significant here that Cicero speaks of the change from du to b as a contraction. He says:
[Cic. Or. LXV.] Quid vero licentius quam quod hominum etiam nomina contrahebant, quo essent aptiora? Nam ut duellum, bellum; et duis, bis; sic Duellium eum qui Poenos classe devicit Bellium nominaverunt, cum superiores appellati essent semper Duellii.
One cannot but feel in reading the numerous passages in the grammarians that treat of the sound of u consonant, that if its sound had been no other than the natural sound of u with consonantal force, they never would have spent so much time and labor in explaining and elucidating it. Why did they not turn it off with the simple explanation which they give to the consonantal i—that of double i? What more natural than to speak of consonant u as “double u” (as we English do w). But on the contrary they expressly declare it to have a sound distinct and peculiar. Quintilian says that even if the form of the Aeolic digamma is rejected by the Romans, yet its force pursues them:
[Quint. XII. x. 29.] Aeolicae quoque litterae qua servum cervumque dicimus, etiamsi forma a nobis repudiata est, vis tamen nos ipsa persequitur.