Classifying Triangles: Equilateral, Isosceles and Scalene

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1 Getting Started
2 Watch and Notice
3 Try It Together
4 Draw One of Each in Your Copy
5 Class Challenge
6 What Did We Notice?
7 What's Next
Pupil practice

1 - Getting Started ~4 mins

Here are three triangles. One has all its sides the same length, one has two sides the same, and one has no two sides the same.

What is the same about all three of them? And what is different?

Teacher Notes

Hold up (or display) three real triangle cut-outs — one all-equal, one with two equal sides, one with none equal. Take two or three hands-up answers, not open call-outs. Steer the class toward noticing that all three have three straight sides and three corners (same) but the side lengths differ (different) — this is the idea the whole lesson hangs on.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~9 mins

Equilateral: all three sides equal

Watch this triangle sit in its family. All three of its sides are the same length, like a yield road sign. We call it equilateral.

Isosceles: two sides equal

This one has exactly two sides the same length, like a slice of toast cut from corner to corner. We call it isosceles (say it "eye-SOSS-uh-leez").

Scalene: no two sides equal

This triangle has all three sides a different length. We call it scalene (say it "SKAY-leen").

A tilted scalene: still the same family

Here is a scalene triangle turned on its side. It still has three different sides, so it is still scalene. Turning a triangle never changes its family.

Teacher Notes

Walk each example aloud, one at a time, and point at the sides as you go. Say isosceles slowly the first time and have the class say it back once.

Equilateral — show that a strip of paper laid along one side reaches exactly along each of the other two. All three match.
Isosceles — find the two that match, then the odd one out. Two match.
Scalene — show that no two sides line up. None match.
Tilted scalene — this is the key idea: it is the same triangle turned, so the family has not changed. Ask 'has anything about the sides changed just because I turned it?'

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~8 mins

Today we sort triangles into their three families. One pupil comes to the board to drag a triangle. Before they drag, everyone watching says which sides they think match — then the pupil drags it to the right family: equilateral, isosceles or scalene.

Sort the triangles into families

Teacher Notes

This round is for talking it through together — pupils take turns at the board and the class agrees or corrects out loud.

Before each pupil drags, have the class say which sides they think match. Use the paper-strip check on screen if a triangle is hard to judge by eye. Rotate enough pupils to work through the bank briskly. Watch for the common slip of counting an equilateral as isosceles (all-equal triangles also have 'two equal sides', but we name them by the strongest match — all three equal).

+15 XP

4 - Draw One of Each in Your Copy ~3 mins

COPYBOOK MOMENT

In your maths copy, draw one triangle of each kind. Label one equilateral, one isosceles and one scalene.

Teacher Notes

Walk the room and glance for three clearly different triangles and the right labels — no marking, this is whole-class copybook practice. Look for the equilateral being drawn too lopsided and the scalene being drawn with two sides that look equal.

+10 XP

5 - Class Challenge ~11 mins

Now we make triangles with strips of paper or lollipop sticks. Work through these in order:

Make a triangle with all three sides equal.
Make a triangle with exactly two sides equal.
Make a triangle with no two sides equal.
Try to make a triangle with sides of 3 cm, 3 cm and 7 cm. The two short sides add up to 6 cm, which is shorter than the 7 cm side, so they cannot reach across to meet. For a triangle to close, the two shorter sides together must be longer than the longest side.

Hands-on Task

Teacher Notes

This round is the practice bank — pupils work the four builds at their desks with strips or sticks, and the class confirms each result aloud before moving on. Keep it brisk rather than over-explaining.

All equal = equilateral; two equal = isosceles; none equal = scalene — name the family each time.
The 3-3-7 build is the stretch: the two short sides cannot reach across the long one, so the triangle cannot close. Have pupils lay the 3 cm and 3 cm strips end to end against the 7 cm strip so they see the gap, then ask 'why won't it close?' before telling them.
Fast finishers: name the family of a triangle they can spot somewhere in the room.

Differentiation: pair a pupil who is less secure with a peer who can model laying the strips end to end carefully (teacher's grouping choice).

+15 XP

6 - What Did We Notice? ~3 mins

MATHS TALK

Can a triangle ever belong to two families at once? Think about a triangle with all three sides equal — does it also have two sides equal?

It does! An all-equal triangle has matching pairs of sides too. But because all three sides match, we give it the most special name, equilateral, not isosceles.

Teacher Notes

Listen for the insight that an equilateral triangle does have two equal sides (it has three) — but we give it the strongest name, equilateral, not isosceles. Revoice a good answer: 'so all-equal beats two-equal — we name it by the most that matches.' Head off the idea that turning a triangle could move it between families.

+10 XP

7 - What's Next ~2 mins

What we learned today

Every triangle has three sides — the side lengths sort it into a family.
All three equal is equilateral, exactly two equal is isosceles, none equal is scalene.
An equilateral triangle does have two equal sides too — but because all three match, we give it the strongest name, equilateral.
Turning a triangle never changes its family.

Coming up

Next we look at four-sided shapes — squares, rectangles, rhombuses and more — and sort them by their sides and corners too.

Teacher Notes

Close briskly. A quick whole-class recall of the three family names by their side rule is enough.

+5 XP

Pupil practice

Module 7 · 2D and 3D Shapes and Angles Data & Chance

Lesson 72 · Classifying Triangles: Equilateral, Isosceles and Scalene

Download Activity Book page (PDF)

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Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~9 mins

Equilateral: all three sides equal

Isosceles: two sides equal

Scalene: no two sides equal

A tilted scalene: still the same family

3 - Try It Together ~8 mins

Sort the triangles into families

4 - Draw One of Each in Your Copy ~3 mins

5 - Class Challenge ~11 mins

Hands-on Task

6 - What Did We Notice? ~3 mins

7 - What's Next ~2 mins

What we learned today

Coming up

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