Plane

Plane, in geometry. From experience we get the idea of a plane as a flat surface which can be extended to infinity in all directions. If we take any two points and pass a plane through them, we can turn this plane about the line joining the two points till it passes through another point. The plane is now fixed; hence, through any three points in space, one, and only one, plane can be drawn. Euclid defines a plane as that in which any two points being taken, the straight line joining them lies wholly in that plane. This is the same as saying that it has no curvature. It is, therefore, the limiting case of a sphere when the radius has become infinite. Two planes intersect in a straight line, and their inclination to each other is measured by the angle between two lines - one in each plane - drawn perpendicular to the line of intersection. When the planes are perpendicular to each other, this angle is a right angle. Any plane divides the whole of space into two parts.