The dotted line through the middle of the letter A is called a mirror line, because if you place a mirror along it, the shape looks exactly the same as the original.
Another name for a mirror line is a line of symmetry. This kind of symmetry can also be called 'reflective symmetry'.
Now look what happens when you fold the letter 'A' along some dotted lines.
Only in the third example does the folded part of the shape fit exactly over the original part. The dotted line is its line of symmetry.
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