or 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 contradistinction 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.

Plain-Sailing. See Navigation, p. 685.