quad
A square space the size of an em that was used in lead typesetting to space out a line to its full justified width so that each letter was held in place when the frame was moved. For example, to quad right, the typesetter placed all the quads on the left side of the line to push the type to the right. Used in photographic typesetting as a placement term. Originally a piece of type metal used to fill out lines and spacing between words.