A variety of symbols have been used to indicate and unknown quantity. The Hindus used , from meaning "so much as," for an unknown in the seventh century. Additional unknowns were indicated by initial syllables of words for different colors. For example, a second unknown might be denoted by , from , "black."
In the late fifteenth and early sixteenth centuries, the Italians indicated an unknown x with co, from cosa, "thing." They used ce, from censo, for x2, cu, from cuba, for x3, and cece, from censocenso, for x4.
The French mathematician Francois Viète (1540-1603) used vowels for unknown quantities and consonants for known quantities. He wrote x as A, x2 as A quadratum, and x3 as A cubum. Later writers shortened these to A, A q, and A c.
In 1637 René Descartes introduced our current custom of using the latter letters of the alphabet for unknowns, and the earlier letters for knowns. He also introduced our x, x2, and x3 used today.