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1Curved vs Straight Lines2Lines, Rays & Line Segments32D Shapes43D Shapes5Triangle Classification6Symmetry7Compound Perimeter

Concept 3 of 7

2D Shapes: Sides & Corners

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Today's big idea

A flat shape is named by counting its straight sides, and its corners always match.

Curves are not sides, which is why a circle sits outside the naming rule.

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How it works

👀 What's going on?

Look around the room and count sides. A window has 4 straight sides. A warning road sign has 3. A plate has none at all, because its whole edge is one smooth curve.

💡 Here's the trick

Name shapes by their sidesTriangle3 sidesSquare4 sidesPentagon5 sidesHexagon6 sidesCircle = no straight sides

Look at the four shapes above. Each one is named by counting its straight sides, and the corners always come to the same total. 3 sides makes a triangle, 4 makes a quadrilateral, 5 makes a pentagon and 6 makes a hexagon. The circle sits apart from all of them, because a smooth curve is not a side.

🪄 How to do it

  1. 1.Go once round the shape above and count every straight side.
  2. 2.Go round again and count the corners, which should give the same number.
  3. 3.Match that number to the name.

✍️ Try it

  1. 1.A stop sign has 8 straight sides. What is that shape called?
  2. 2.A shape has 7 corners and every one of its sides is straight. How many sides has it, and what is it called?
  1. 1.8 straight sides gives the name octagon.
  2. 2.Corners and sides always come in equal numbers, so 7 corners means 7 sides. A 7-sided shape is called a heptagon.

⚠️ Watch out

You might be thinking that a square and a rectangle are two different shapes that can never be the same. A rectangle is any 4-sided shape whose corners are all square corners. A square has 4 square corners, so every square is a rectangle as well. A square is also a rhombus, because all 4 of its sides are equal.

🌟 Why this matters

Floor tiles, road signs and window frames are chosen by their number of sides, because that decides how they fit together. Hexagons tile a floor with no gaps, and circles never will. Naming a shape by its sides is the quickest way to say what it can do.
Exam tip: Class 3 olympiad questions often hide the name inside a corner count instead of a side count.

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Watch me solve one

Quanta solves it

“Watch how I think through one of these — then you try.”

THE PROBLEM

“A shape has 6 equal straight sides. What is it called?”

  1. 1

    Count the sides

    The shape has 6 straight sides. 🔺

  2. 2

    Recall the side → name

    3=triangle · 4=quad · 5=pentagon · 6=hexagon.

  3. 3

    Check 'equal'

    All sides equal → REGULAR hexagon. Bees build these!

  4. 4

    Answer ✓

    The shape is a hexagon

Now you try one — same steps, different story 👇

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See it in action

⚡ Time to play!

Tap each corner to count them. How many corners does this shape have?

2D SHAPES · ROUND 1 OF 3

Corners: 0
5

Quick check

✅ VERIFICATION

A shape has 4 straight sides, and not one of its corners is a square corner. Which name still fits it?

⚡ Finish the activity above first to answer.

Keep going!

Up next: 3D Shapes

Cubes, cones, spheres, count what each has.

Concept 4 of 7 →