Classifying polygons
A polygon is a closed shape with straight sides. To classify a polygon, count its sides.
The names
| Sides | Name |
| 3 | triangle |
| 4 | quadrilateral |
| 5 | pentagon |
| 6 | hexagon |
| 7 | heptagon |
| 8 | octagon |
The Greek prefix tells you the count: tri- 3, quad- 4, penta- 5, hexa- 6.
Special quadrilaterals
A quadrilateral has 4 sides. The most important kinds:
- Square — 4 equal sides, 4 right angles.
- Rectangle — opposite sides equal, 4 right angles.
- Rhombus — 4 equal sides (no right angles needed).
Squares are also rectangles AND rhombuses (a square is a "perfect rectangle" and a "perfect rhombus").
Triangles by sides
- Equilateral — all 3 sides equal.
- Isosceles — 2 equal sides.
- Scalene — no equal sides.
Watch out — closed and straight
For a shape to be a polygon:
- it must be closed (no gaps),
- all sides must be straight (no curves).
A circle is not a polygon.
Try it
Summary
- Polygon = closed shape with straight sides.
- Name by side count: 3 = triangle, 4 = quadrilateral, 5 = pentagon, 6 = hexagon.
- Squares and rectangles are special quadrilaterals.