A regular decagon has 10 lines of symmetry, one line drawn through each vertex and one line drawn through the midpoint of each edge. A line of symmetry indicates where a polygon can be folded in half to form two equal shapes.

The number of lines of symmetry in regular polygons is equal to the number of sides of the polygon. A regular decagon is a two-dimensional polygon with 10 sides of equal length, so it has 10 lines of symmetry. Most irregular polygons do not have a line of symmetry, with the isosceles triangle and rectangle serving as two exceptions.