Triangle, three-sided polygon. There are three main types of plane triangle: scalene, in which no side is equal in length to another; isosceles, in which two of the sides are equal in length; and equilateral, in which all three sides are equal in length. A right (or right-angled) triangle has one interior angle equal to 90° and may be either scalene or isosceles. The “corners” of a triangle are termed vertices (singular: vertex). The sum of the angles of a plane triangle is 180°. A spherical triangle is an area of the surface of a sphere bounded by arcs of three great circles, each arc being less than 180°, each side and interior angle being termed an element. The sum of the three sides is never greater than 360°; the sum of the three angles is always in the range of 180°–540°.