(Two-dimensional) shapes that are symmetrical can be divided into two equal parts by a straight line, such that, if the shape were to be reflected in that line, the two parts would reflect on to each other.
The last part of that is important to understand - for example, a square has two diagonal lines of symmetry - reflecting the square along this line of symmetry would result in the square appearing unmoved (some of its vertices would have moved); however this is not true for the diagonal line of a rectangle - try it by folding a piece of A4 paper along its diagonal - the two halves of the paper will not sit on top of each other!
Regular polygons are symmetrical - they will have a number of lines of symmetry, the same as their number of sides/angles. So a pentagon will have five lines of symmetry, a hexagon six, and so on. Identifying symmetry in shapes allows us to solve problems involving angles and side lengths in geometry.
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