A shape has line symmetry if it can be folded in half along a line so both halves match exactly. A square has 4 lines of symmetry; a rectangle has 2. Students identify and draw lines of symmetry using folding and mirrors.
You already know the attributes of 2D shapes — how many sides they have, whether sides are equal, whether angles are right angles. Line symmetry (also called reflective symmetry) adds a new kind of attribute: whether a shape can be folded along a line so that both halves land exactly on top of each other. The fold line is called a line of symmetry, and any point on one side of the line has a matching point at the exact same distance on the other side.
The most reliable way to find a line of symmetry is to imagine folding. If you fold a square in half diagonally, the two triangular halves stack perfectly — so the diagonal is a line of symmetry. A square has 4 lines of symmetry in total: two diagonals and two lines through the midpoints of opposite sides. A rectangle only has 2 lines of symmetry (both through midpoints of opposite sides), because folding it diagonally gives two halves that don't match up — the corners don't align.
Not every shape has a line of symmetry. A scalene triangle (with all different side lengths) has none. The letter Z has none. The letter A has one vertical line of symmetry. A regular hexagon has six. In general, the more equal a shape's sides and angles are, the more lines of symmetry it tends to have. Counting lines of symmetry is connected to what you know about regularity in shapes.
The practical test is always to fold (or place a mirror along the proposed line) and check. If the two halves don't stack perfectly, the line you drew is not a line of symmetry. The key requirement is that both halves must be exactly the same shape and size — not just similar-looking. Symmetry is about perfect balance, and the line of symmetry is exactly the dividing line where that balance holds.