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Sorted by rules

Why Do Shapes Have Names?

A shape's name is a checklist. Meet the rules and you get the name.

13 straightsides —triangle24 equal sides,4 right angles— square3Every pointequally farfrom the centre
Names follow rules, not appearance.

  1. Step 1 of 5

    Names are rules, not looks.

    Not a triangleCurved sidesTriangleThree straight sides,closed

    A triangle is not 'a pointy thing'. It is any closed shape with exactly three straight sides. Long, thin, wonky — still a triangle.

  2. Step 2 of 5

    Count the sides to find the family.

    TriangleQuadrilateralHexagon

    Three sides makes a triangle, four a quadrilateral, five a pentagon, six a hexagon. The Greek number word is right there in the name.

  3. Step 3 of 5

    Extra rules make smaller families.

    Quadrilateral — 4 sidesthe widestfamilyRectangle — plus 4 rightanglesnarrowerSquare — plus 4 equal sidesnarrowest

    All four-sided shapes are quadrilaterals. Add 'opposite sides parallel' and you get a parallelogram. Add 'all sides equal and all angles right' and you have a square.

  4. Step 4 of 5

    So a square is also a rectangle.

    Every squareIs a rectangleNot every rectangle Is a square

    It meets every rule a rectangle has, plus one more. Squares are a special kind of rectangle, the way a robin is a special kind of bird.

  5. Step 5 of 5

    A circle plays by different rules.

    A circle Every edge point equallyfar from the middle

    It has no sides and no corners. Its rule is about distance: every point on the edge is exactly the same distance from the centre.

    This is why wheels are round. A constant distance from the centre means the axle never bounces up and down.

The short version

A shape's name is a checklist of rules, and meeting a stricter checklist puts you in a smaller family.

Try it yourself

Draw the strangest four-sided shape you can. It is still a quadrilateral — the rules only ask for four straight sides.

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