Why do we have the letter "C" in English?
“C” is more common than “K” by the way. I know that from my job at a reading school and my own research. “C” is most often pronounced like “K” but borrows from “S” when it’s followed by “I, e,” or “y” (an exception being the word soccer). Really the only unique contribution “C” makes to our language is when it’s paired with “H” as in “ch” (but can sound like “K” in words of Greek origin: anchor, or “sh” in words of French origin: chef). Occasionally it breaks the rules when paired with “E” or “I” as in “ocean” or “glacier”.
Sometimes it’s silent: muscle.
“C” and “G” are very closely related. Not just in shape, of course, but in sound as well. Physically, the tongue moves the same way to produce both sounds. The difference is that “G” is voiced, and “C” is not (the vocal chords vibrate to produce the noisy “G” sound). “G” also can turn soft (make the “J” sound) when “I,e” and “Y” come after it (but it doesn’t have to).
Alphabets came from previous alphabets, borrowing letters. The first was Egyptian, and going down the branch that led to English, next came the Phoenician alphabet, then the Greek, then the Etruscan, and then the Roman, whose letters we use.
3,000 years ago, Phoenician’s alphabet had “G” as the third letter. “C” wasn’t around yet. The Greeks copied their alphabet and brought it to Italy. The Etruscans there had no “G” sound, apparently. Their nearest sound to it was “K”. (So they didn’t need Greek’s third letter “G” [gamma]). They had “K” and “Q”. Sources show that the Etruscans kept gamma in third place in the alphabet but used it to represent the sound “k”. The shape of the letter went through changes, and the Etruscans stopped using the Greek name gamma and went to something similar to “kay”. They might have said the spelling was “C-E” (long e sound).
Romans, Latin speakers, probably didn’t like the Etruscan alphabet with three letters making a “K” sound and none for “G”. They made their own letter for “G” and used “C” more often than “K”. Still “C” only represented the “K” sound. (It’s interesting to note, according the book mentioned above, that Caesar was pronounced like “Kye-sar”.)
When did “C” start borrowing from “S”? In later Roman times. Everyday Latin speakers began to slur the Latin “C” before “I,e” and “Y” (vowels where the tongue is pushed forward) and making it sound like “ch”. This sound became part of languages such as French and Spanish etc. (languages arising from the dying Latin).
“C” in the Middle Ages sounded like “K” and “ch”. Italian’s soft “C” is “ch”. As for the other Romance languages, the sound disintegrated to “s”.
Remember reading about the Norman invasion of England in 1066? Those Normans brought their Medieval French there, and it mixed with Old English eventually melding into an interesting Middle English. So, most of our words with the soft “C” sound came from that Norman French. Our newer words like “cybernetics” follow those old rules.