Plane, in general, an appellation given to whatever is smooth and even, or simple, obvious, and easy to be understood; and, consequently, stands opposed to rough, enriched, or laboured.

A plain figure, in geometry, is an uniform surface; from every point of whose perimeter right lines may be drawn to every other point in the same.

A plain angle is one contained under two lines, or surfaces, in contradiction to a solid angle. See Angle.

The doctrine of plain triangles, as those included under three right lines, is termed plain trigonometry. See the article Trigonometry.

Plain Chart. See the article Chart.