I don't remember who it was, but a while back we were talking about ways to spell /k/, and someone said <c> and <qu> don't alternate word- initially in Spanish. I meant to point out that there is at least one word in Spanish where the two alternate word-initially: _caber_ "to fit, admit, allow". The 1st-person singular indicative is _quepo_, and the present subjunctive and the imperatives in the 2nd singular/plural formal and 1st plural follow suit. Diachronically this vowel change is due to a survival of the /j/ in the Latin endings, which metathesized, e.g.: /kapjo/ > /kajpo/ > /kejpo/ > /kepo/. The glide also keeps the / p/ from leniting; compare /sapja/ > /sepa/ (from _saber_).