What is a 2D shape?
A 2D shape is a form drawn flat, on a page: it only has two dimensions (length and width), no thickness. It's the opposite of a 3D shape, which exists in space, with volume.
The most common 2D shapes are the square, the rectangle, the triangle, the circle and the rhombus.
Sides and vertices
To describe a 2D shape, we count two things:
- Sides: the straight-line segments that form its outline.
- Vertices: the points where two sides meet, at each "corner" of the shape.
A square has 4 sides and 4 vertices. A triangle has 3 sides and 3 vertices. The number of sides and vertices is always the same in a closed shape.
The circle
The circle is different: it has no sides and no vertices, its outline is a single curved line. It's defined by its centre (the point in the middle) and its radius (the distance from the centre to any point on the circle).
- The radius joins the centre to a point on the circle.
- The diameter crosses the circle right through, passing through the centre — it's always twice the radius.
diameter = 2 × radius
The different triangles
Every triangle has 3 sides, but they're grouped into families by their side lengths and angles.
- Equilateral triangle: all 3 sides are exactly the same length (and so are all 3 angles).
- Isosceles triangle: at least 2 sides are the same length.
- Right-angled triangle: it has a right angle (90°).
- Scalene triangle: no side is the same length as another.
The different quadrilaterals
A quadrilateral is a shape with 4 sides. The best-known ones have precise properties:
- Square: 4 equal sides, 4 right angles.
- Rectangle: 4 right angles, opposite sides equal in pairs (but not necessarily all equal to each other).
- Rhombus: 4 equal sides, but its angles aren't necessarily right angles.
- Parallelogram: opposite sides parallel and equal in pairs, with no right angle required.
- Trapezium: only two opposite sides are parallel (not all four).
How to recognise a shape properly
- Count the number of sides (3 = triangle, 4 = quadrilateral, curved = circle).
- Check whether any sides are the same length (with a ruler, or by eye).
- Check whether the shape has any right angles (with a set square, or the corner of a piece of paper).
- Combine these clues to find the exact name of the shape.
Mistakes to avoid
- Thinking a rhombus necessarily has right angles — only the square (which IS a special rhombus) does.
- Confusing a rectangle and a square: every square is a rectangle, but not the other way round.
- Forgetting that a square is also a rhombus, thinking they're two completely separate families.
- Mixing up a circle's radius and diameter (the diameter is twice the radius).
Worked exercises
A triangle has sides of 5 cm, 5 cm and 5 cm. What is it called?
A shape has 4 equal sides, but no right angles. What is it?
A circle has a radius of 4 cm. What is its diameter?
Now it's your turn!
On EasyMaths, your child practises recognising and classifying 2D shapes with exercises that never repeat and instant marking.
Start the exercises →Frequently asked questions
What's the difference between a side and a vertex?
A side is a segment (a straight line) that forms the outline of the shape. A vertex is the point where two sides meet, at a corner of the shape. A square has 4 sides and 4 vertices.
How do you recognise an equilateral, isosceles or right-angled triangle?
Equilateral: all 3 sides are the same length. Isosceles: at least 2 sides are the same length. Right-angled: it has a right angle. A triangle with none of these properties is called scalene.
What's the difference between a square and a rectangle?
A rectangle has 4 right angles, with opposite sides equal in pairs. A square is a special type of rectangle where all 4 sides are exactly the same length — so a square is always a rectangle, but a rectangle isn't always a square.