Regular polytope impossible ("not constructible"). The simplest few regular polygons that are impossible to construct are the "n"-sided polygons with "n" equal to 7, 9, 11, 13, 14, 18, 19, 21... Constructibility in this sense refers only to ideal constructions with ideal tools. Of course reasonably accurate approximations can be constructed by a range of methods; while theoretically possible constructions may be impractical. Euclid's "Elements" gave what amount to ruler-and-compass constructions for the five Platonic solids. (See, for example, Euclid's Elements.) However, the merely practical question of how one might draw a straight line in space, even with a ruler, might lead
Regular polygon (of order 2"n"): "D", "D", "D", ... It consists of the rotations in "C", together with reflection symmetry in "n" axes that pass through the center. If "n" is even then half of these axes pass through two opposite vertices, and the other half through the midpoint of opposite sides. If "n" is odd then all axes pass through a vertex and the midpoint of the opposite side. All regular simple polygons (a simple polygon is one that does not intersect itself anywhere) are convex. Those having the same number of sides are also similar. An "n"-sided convex regular polygon