The letter K comes from the Greek letter Κ (kappa), which was taken from the Semitic kap, the symbol for an open hand. This, in turn, was likely adapted by Semites who had lived in Egypt from the hieroglyph for «hand» representing D in the Egyptian word for hand, d-r-t. The Semites evidently assigned it the sound value /k/ instead, because their word for hand started with that sound.

In the earliest Latin inscriptions, the letters C, K and Q were all used to represent the sounds /k/ and /g/ (which were not differentiated in writing). Of these, Q was used to represent /k/ or /g/ before a rounded vowel, K before /a/, and C elsewhere. Later, the use of C and its variant G replaced most usages of K and Q. K survived only in a few fossilized forms such as Kalendae, «the calends».

After Greek words were taken into Latin, the Kappa was transliterated as a C. Loanwords from other alphabets with the sound /k/ were also transliterated with C. Hence, the Romance languages generally use C and have K only in later loanwords from other language groups. The Celtic languages also tended to use C instead of K, and this influence carried over into Old English.