A line of symmetry divides a shape into two matching parts, which will overlap each other exactly when folded along the line of symmetry.

Look at this **isosceles trapezium**.

When we reflect or fold it in the **line a**, we get:

When we reflect it in the **line b**, we get:

This is a **MATCH**!

When we reflect it in the **line c**, we get:

When we reflect it in the **line d**, we get:

It is **only **in line b that we get a match.

So **line b **is the only line of symmetry.