[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:

[Quint. I. iv. 9.] Et K, quae et ipsa quorundam nominum nota est.

And: