The unit of measurement for types, leads, rules, and other small items used in composing a page is the point, approximately 1/72 of an inch, shown by the thickness of this mark:" The most used type bodies are those of 6-point and others graduated by one point up to 12-point.

The amount of type in a page is measured in ems of the size of type used. An em is a square of the body em-square, and varies in size with each size of type. Thus, an 8-point em is 8points deep and 8points wide; a 12-point em is 12points deep and 12points wide. The common method of measuring the quantity of type on a page is by using the em as the unit, the number of ems in the line being multiplied by the number of lines on the page. The term em is applied in many ways to type; the em dash is one cast on a square body, the em fraction is a fraction cast on a square body, and so with type borders and other characters.

Before the adoption of the point system type sizes were designated by a variety of names which were meaningless so far as indicating their sizes was concerned. In the point system the size of 12-point corresponds to the old pica. Pica has been a standard type in many countries for a long time, though it has not always been uniform in size. All type founders made pica types, but all picas were not the same size in this country until after the adoption of the point system in 1887. The old names pica and nonpareil (half pica, or 6-point) still survive as convenient terms to use in naming these sizes.

For convenience and economy in the composing room the leads, slugs, rules, metal and wooden furniture, wood type, and other composing material are used in lengths graduated by 12-point or pica. When leads or rules are spoken of as being twenty picas, or twenty ems, it is understood that they are twenty 12-points long. Pica gages are scales marked off in units of 12-point (and half, or 6-point). A graduated composing stick is made to set to measures of 12-point and half.

It will be remembered that the point size was given as approximately 1/72 of an inch. Actually a point is .013837 of an inch, but for convenience the simpler fraction of 1/72 is sufficiently accurate for composing-room purposes. This makes the 12-point or pica 1/6 of an inch. Shop custom measures the items of a page in points, and the page itself or its chief divisions by picas. Paper, sizes and other large dimensions are measured in inches.

It is important for the apprentice to learn these units of measurements and their relation to each other in order to make quick calculations for line lengths, page sizes, margins, etc.