Perpendicular. 1. In Architecture, a style of Gothic which prevailed in England in the 15th and 16th centuries, and was contemporary with the French Flamboyant style, though not possessing its characteristic ornamentation. It is pointed Gothic, and its chief feature is the prevalence of straight vertical lines. Its windows bad vertical mullions and horizontal transoms, and are noted for their large size. Other features are vaulted roofs with fan tracery, such as may be seen in King's College Chapel, Cambridge; open timber roofs, such as that of Westminster Hall; and panelling, such as may be seen in some of the colleges of Oxford and Cambridge, and in some of our cathedrals. 2. In Geometry, one line is said to be perpendicular to another when it makes equal angles with the other on both sides. A line is perpendicular to a plane when it makes a right angle with two lines in that plane, and in this case it can be proved that it is also at right angles to every line in the plane. Two planes are perpendicular to each other when any line in one plane perpendicular to their line of intersection is at right angles to the other plane. The word "perpendicular" is sometimes popularly used to mean vertical.