The different letters of the alphabet vary in thickness. The _m_, which, whether capital, lowercase, or italic, is nominally square in body,--that is, just as broad as the line is deep,--is taken in America as the basis of measuring the quantity of matter in a page, and, thus used, is written "em." The unit of measurement is a thousand "ems," which means an amount of matter equal to a thousand such square types. A line of this article measures seventeen ems, and there are fifty-five lines to a column, thus a full page contains 1,870 ems, for which a compositor would usually be paid forty-four cents. Every one who reads knows that some letters are used more frequently than others. For the ordinary class of English work, the relative ratios of the letters, as nearly as can be calculated, are as follows :--y, l, k, j, q, x--3; b, v--7; g, p, w, y--10; c, f, u, m--12; d, l--20; h, r--30; a, i, n, o, s--40; t--45; e--60; in all, 532. The "fonts," or supplies of single styles of type, are made of all sizes, from two or three pounds to thousands of pounds, according to the quantity needed. Before the types are used they are placed in two "cases," called respectively the "upper" and "lower," which are placed on a stand or "frame." The upper case is divided into ninety-eight boxes of equal size, in which are placed the CAPITAL and SMALL CAPITAL letters, as in the plan given, by which the position of each letter and character may be seen. The lower case has fifty-four compartments of different sizes, in which are the "lowercase" letters, spaces, quadrats--commonly called "quads"--and other prime necessities for a printing office. The quadrats are pieces of metal lower than the type, and are used for filling out blank spaces, such as the incomplete lines at the end of a paragraph, while the "spaces," which vary from the thickness of a hair to the width of the letter _n_, make the spaces between words. The larger spaces are all multiples of the _m_, which is square, and are therefore called quadrats, or quads. [Illustration: MAKING "PI."] With a pair of these cases before him, the compositor begins his work. His "copy" (the reading matter to be set in type) lies before him on the right hand side of the upper case, which is very seldom used. He has in his mind a phrase of the article he is setting, and picks up the letters one by one, placing them in turn in a composing "stick," which he holds in his left hand. He does not pick the letters from their boxes at random, but, as a matter of habit, his eye searches out a particular letter that lies in a position to be grasped before his hand reaches it. He never looks at the face of a letter to be sure of what it is, but only at the notch, or "nick," at one side at the bottom, which must invariably be placed upward or towards his thumb in the stick. With the nicks down the words would look as follows: [Illustration: BACKWARD WRITING.]