The particular letter assigned to represent a particular quantity is a matter of no consequence. Any letter at will may be taken; but when taken, it must be firmly adhered to to represent that particular quantity, throughout all the modifications which may be requisite in condensing the statement into which it enters into a formula for use. For example, the two rules named in Art. 399 are precisely alike - three quantities given to find a fourth - yet they are represented by different letters. In one, R and P represent the two weights, and m and n the arms of leverage at which they act; while in the other the letters a, b,c, and d represent severally the four lines which constitute two similar sides of two homologous triangles. The two rules are alike in working, and they might have been constituted with the same letters. And instead of the letters chosen any others might have been taken, which convenience or mere caprice might have dictated. In some questions it is usual to put the first letters, as a, b, c, etc., to represent known quantities, and the last letters, as x, y, z, for the quantities sought. In works on the strength of materials it is customary to represent weights by capital letters, as P, R, U, W, etc., and lines or linear dimensions by the small letters, as b, d, l, for the breadth, depth, and length, respectively, of a beam. Any other letters may be put to represent these quantities, although the initial letter of the word serves to assist the memory in recognizing the particular dimensions intended.