🔢 MathLevel 1Lesson 18
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Properties of 2D Shapes — Sides and Corners

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What you will learn

  • 1Count the sides and corners of a 2D shape
  • 2Use sides and corners to sort and classify shapes
  • 3Recognize that the number of sides equals the number of corners
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Mini Lesson

Last lesson we learned the names of 2D shapes. Now let's look more closely — what makes each shape different? The answer is in the sides and corners!

What are sides and corners?

  • A side is a straight line that forms the edge of a shape.
  • A corner (also called a vertex) is the point where two sides meet.
  • Every 2D shape (except the circle) has the same number of sides and corners.

How to count sides correctly

  • Start at one corner and mark it with your finger.
  • Count each line as you move around the shape: 1, 2, 3…
  • Stop when you get back to where you started.

Sorting shapes by sides

  • 3 sides → triangle (any triangle — tall, flat, big, or small)
  • 4 sides → square, rectangle, rhombus (all have four sides)
  • 5 sides → pentagon
  • 6 sides → hexagon (shapes with 5 or more sides go in the 5+ sides group)

Words to know

Side

A straight line that forms the edge of a shape

e.g. A square has 4 sides.

Corner

The point where two sides of a shape meet

e.g. A triangle has 3 corners.

Vertex

Another word for a corner of a shape

e.g. Each corner of a square is a vertex.

Property

Something true about a shape, like its number of sides

e.g. A property of a hexagon is having 6 sides.

Sort

To put shapes into groups that match

e.g. I sort shapes by how many sides they have.

Sides and corners labeled on a triangle and a rectangleA green triangle on the left with its three sides and three corners numbered. A blue rectangle on the right with its four sides and four corners numbered.Sides and Corners1233 sides,3 corners13424 sides, 4 corners
A triangle with 3 labeled sides and 3 orange corners; a rectangle with 4 labeled sides and 4 orange corners.
Shapes sorted into three groups by number of sidesThree labeled columns: 3 sides shows a triangle, 4 sides shows a square and a rectangle, and 5 plus sides shows a pentagon and a hexagon.Sort by Sides3 sides4 sides5+ sidesTriangleSquareRectanglePentagonHexagon
Three sorting columns — 3 sides (triangle), 4 sides (square, rectangle), 5+ sides (pentagon, hexagon).
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Ask yourself: "How many sides does this shape have? Which group does it belong to?"

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← Lesson 17: Naming 2D ShapesLesson 19: Naming 3D Shapes →