A line of symmetry is a line through a shape such that folding along the line would make the two halves match exactly.
How to find one
Imagine folding the shape in half along a line.
If both halves overlap perfectly, that line is a line of symmetry.
If they don't, it's not.
How many lines does a shape have?
Shape
Lines of symmetry
Square
4
Rectangle
2
Equilateral triangle
3
Isosceles triangle
1
Scalene triangle
0
Circle
infinite
Regular pentagon
5
Regular hexagon
6
A regular polygon (all sides and angles equal) has as many lines of symmetry as it has sides.
Where the lines are
For a square, the 4 lines run:
vertically (top to bottom),
horizontally (left to right),
corner to corner (both diagonals).
For a rectangle, the 2 lines run vertically and horizontally through the centre — but not diagonally, because the diagonal halves wouldn't overlap (the rectangle isn't square).
Real-life symmetry
Most letters of the alphabet — A, H, I, M, O, T, U, V, W, X, Y — have at least one line of symmetry.
Butterflies have 1 line (down the middle of the body).
