# Definition 19

*Rectilinear figures* are those which are contained by straight lines, *trilateral* figures being those contained by three, *quadrilateral* those contained by four, and *multilateral* those contained by more than four straight lines.

## Guide

This definition was probably not in Euclid’s original *Elements*.
It classifies rectilinear figures by their number of sides. Euclid names polygons by the number of their angles. For example, Book IV includes constructions of regular pentagons, hexagons, and pentadecagons (15-angled figures) in which those terms are used.

The modern English names are also based an the number of angles (except quadrilateral): triangle, pentagon, hexagon, heptagon, octagon, etc. Quadrilaterals can also be called tetragons. From pentagon on up these names derive from the Greek, but they’re rarely used past octagon.