Prefixes in words for geometrical figures
The prefixes in English words for two- and three-dimensional geometrical figures generally derive from the Greek words for the numbers. See Greek and Latin words for numbers. There are some exceptions - in the case of the words for two-dimensional figures with three, four, or nine sides, as noted in the first table below.
As you will see, the words for two-dimensional (i.e., plane) figures are usually formed by adding the prefix to the stem -gon. This stem comes from the Greek word gonia, which means 'corner or angle'. The words for three-dimensional (i.e., solid) figures are formed by adding the prefix to the stem -hedron. This stem comes from the Greek word hedra, which means 'seat or base'.
| No. of sides | Name of figure | Comments |
|---|---|---|
| 3 | triangle | This derives from the Latin tres (three) and angulus, meaning 'corner or angle'. The Greek-derived 'trigon' is archaic. |
| 4 | quadrilateral | This derives from the Latin quattuor (four) and latus/lateris, meaning 'side'. The Greek-derived 'tetragon' is archaic. |
| 5 | pentagon | - |
| 6 | hexagon | - |
| 7 | heptagon | - |
| 8 | octagon | - |
| 9 | nonagon | 'nona-' comes from the Latin nonus, meaning 'ninth'. The Greek-derived 'enneagon' is also possible. |
| 10 | decagon | - |
| 11 | hendecagon | - |
| 12 | dodecagon | - |
| No. of faces | Name of figure | Comments |
|---|---|---|
| 4 | tetrahedron | A pyramid is one type of tetrahedron. A regular tetrahedron has four plane sides which are equilateral triangles. |
| 5 | pentahedron | - |
| 6 | hexahedron | A regular hexahedron is a cube. |
| 7 | heptahedron | - |
| 8 | octahedron | - |
| 9 | enneahedron | - |
| 10 | decahedron | - |
| 11 | hendecahedron | - |
| 12 | dodecahedron | - |
| 20 | icosahedron | - |
See also Greek and Latin words for numbers, Prefixes in units of measurement, Prefixes for very high numbers, and Greek or Latin numerical prefix?.
