The term justifying refers to the tightening of a line to make it correspond with other lines or parts of the page, so that the whole form may be locked together compactly, with no parts loose and none too tight. The term spacing refers to the blanks between the words in the lines. The term leading refers to the distance between the lines in a paragraph or page of plain matter.

A line of type may be well spaced but improperly justified. On the other hand, it may be badly spaced but nicely justified.

There are many places where spaces thinner than the 5-space are needed, especially for letter-spacing and for careful word-spacing, as well as for tabular pages and other work requiring accurate justification. For such purposes hair spaces, copper-thins, and brass-thins are provided. The thickness of hair spaces varies according to the size of the type; the name is given generally to any cast space thinner than the 5-to-em. Copper spaces are 1/2-point in thickness and brass spaces are 1-point thick, the difference in the metal used being for easy distinction of the thicknesses. Copper and brass spaces, because of greater durability, are superior to cast metal hair spaces for hand composition.

When metal thin spaces are not at hand it may be necessary to resort to pieces of paper or thin card. Spaces of this kind should be used only in exceptional cases and not at all as a common practice. There should be never more than a few pieces of paper used in justifying a line. A safe rule for the apprentice, when he thinks he cannot make his line come right without some such expedient, is to ask an experienced compositor, who will usually show him how to justify without the paper.