Obsidian.—A form of lava.
On Centres.—In spacing studs or strips of any kind, they are spoken of as being placed a certain number of inches on centres. E. G. Twelve inches on centres means that the distance from the centre of one piece to the centre of another is twelve inches.
Out of Plumb.—Not vertical; not perpendicular to the horizon.
Out of Square.—Askew; oblique; not true.
Parallel.—Lines or objects are parallel to one another when they have the same direction, and all corresponding points are equidistant from one another.
Perpendicular.—Vertical; plumb; in an upright position.
Perspective.—A perspective is a drawing showing an object in the form in which it appears to the eye. The lines converge, or approach nearer together, as they go away from the eye, until they finally meet at a point on the horizon, or a line drawn to represent the horizon. E. G. In looking down a railway track the telegraph poles and tracks appear to run together on the line of the horizon.
Pica.—The standard of measurement in printing. A name given to a size of type which measures one-sixth of an inch high (measured from the nicked side to the side opposite); and seventy-two ems (see [Em]) measure one foot.
Pie.—A mixed assortment of type. See page [135], Chapter IX.
Pieing Type or Making Pie.—Spilling or mixing up type.