Sides Polygon Name
5 pentagon
6 hexagon
7 heptagon
8 octagon
9 nonagon or enneagon
10 decagon
11 hendecagon or undecagon (even less frequently as unidecagon)
12 dodecagon
13 tridecagon or triskaidecagon
14 tetradecagon or tetrakaidecagon
15 pentadecagon or pentakaidecagon
16 hexadecagon or hexakaidecagon
17 heptadecagon or heptakaidecagon
18 octadecagon or octakaidecagon
19 enneadecagon or enneakaidecagon
20 icosagon
30 triacontagon
40 tetracontagon
50 pentacontagon
60 hexacontagon
70 heptacontagon
80 octacontagon
90 enneacontagon
100 hectogon
10,000 myriagonSome texts list a two-sided polygon as a “digon,” but this is only meant for theo-
retical mathematics.
What are trianglesand how are they classified?
Triangles (or “three angles”) are polygons with three sides. A triangle’s three line seg-
ments (or sides) are joined together at three vertices (endpoints). For all triangles, the
sum of a triangle’s three interior angles is equal to a straight angle, or 180 degrees.
Triangles are classified by either the lengths of their sides, or, more commonly,
the measurement of their angles. All triangles have at least two acute angles, but the
third angle, which can be used to classify the triangle, can be acute, right, or obtuse
(for more about these angles, see above). The types of triangles based on angles are:
Right triangle—There is one angle of 90 degrees. 181