A shape or a pattern has line symmetry when you can fold it in half and both halves exactly overlap.

You can also use a mirror to look for line symmetry. If you place the mirror so that it passes through the centre of the shape or pattern and the reflection is exactly the same as the part of the shape hidden by the mirror then that is a line of symmetry. Shapes can have 0 to many lines of symmetry. An isosceles triangle has 1 line of symmetry, rectangle has two lines of symmetry a square has 4 lines of symmetry.