Diagonal

Diagonal, the line joining opposite corners of a polygon. It may be readily determined when the polygon is regular and with an even number of angular points, but if the figure is complex the rule is to join those corners which are separated by half the number of sides possessed by the polygon. When the number of sides is odd, the half is taken to the nearest figure; thus a diagonal of a heptagon, with seven sides, joins points separated by three sides travelling one way, and by four travelling the reverse way, round the figure.