The grammarians generally agree that k is a superfluous, or at least unnecessary, letter, its place being filled by c. Diomedes says:
[Keil. v. I. pp. 423, 424.] Ex his quibusdam supervacuae videntur k et q, quod c littera harum locum possit implere.
And again:
K consonans muta supervacua, qua utimur quando a correpta sequitur, ut Kalendae, caput, calumniae.
Its only use is as an initial and sign of certain words, and it is followed by short a only.
Victorinus says:
[I. iii. 23.] K autem dicitur monophonos, quia nulli vocali jungitur nisi soli a brevi: et hoc ita ut ab ea pars orationis incipit, aliter autem non recte scribitur.
Priscian says:
[Keil. v. II. p. 36.] K supervacua est, ut supra diximus: quae quamvis scribetur nullam aliam vim habet quam c.
And Quintilian speaks of it as a mere sign, but says some think it should be used when a follows, as initial: