Note:  Do not rely on this information. It is very old.

C

C. The letter C is derived from an earlier form of the Latin G, which was used indifferently to express the sound of G and K in Latin till about 230 B.C. After that, C was used, probably, only to express the sound of K. In English it at first had only this sound, and in Welsh spelling it still retains it exclusively: but when (about the 10th century), the K sound in some English and French words became modified into a sound resembling ts, C also was used for it. In modern English it is used before E and I to express the sound of S. As a numeral in the Roman system it represents 100. In music C is the keynote of the "natural" scale.