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All 2-D shapes are flat. They can have any number of sides that meet up with no gaps.

Some have straight sides.

Some have curved sides.

And some have both straight and curved sides.

2D shapes with all sides equal in length and all angles equal are called regular shapes.

Polygons are 2D shapes with straight sides.

A triangle is a 2-D shape with 3 straight sides.

There are 4 types of triangles: scalene, right-angled, isosceles and equilateral.

Scalene triangle

• No sides equal in length
• All angles different
• No lines of symmetry

Right-angled triangle

• One angle equal to 90°

Isosceles triangle

• Two sides equal in length
• Two angles equal in size
• One line of symmetry

Equilateral triangle

• Regular triangle
• All sides equal in length
• All angles equal to 60°
• Three lines of symmetry

The sum of the angles in any triangle is 180°.

If 2 of the angles in a triangle are known the 3rd angle can be calculated in two steps:

• add together the other 2 angles
• subtract the total from 180

A quadrilateral is any 2-D shape with 4 straight sides.

Some quadrilaterals have special properties.

Square

• Regular quadrilateral
• All sides equal in length
• All angles equal to 90°
• Opposite sides are parallel
• Four lines of symmetry

Rectangle

• Opposite sides equal in length
• All angles equal to 90°
• Opposite sides are parallel
• Two lines of symmetry

Parallelogram

• Opposite sides equal in length
• Opposite angles equal in size
• Opposite sides are parallel
• No lines of symmetry

Rhombus

• All sides equal in length
• Opposite angles equal in size
• Opposite sides are parallel
• Two lines of symmetry

Trapezium

• One pair of opposite sides is parallel

The sum of the angles in any quadrilateral is 360°.

If 3 of the angles in a quadrilateral are known the 4th angle can be calculated in two steps:

• add together the other 3 angles
• subtract the total from 360