What's on one side is the same as what's on the other, reflected.
For example, average human has line of symmetry down the middle. Left eye matches right eye. Left arm matches right arm, and so on.
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2 lines of symmetry
A nephroid has 2 lines of symmetry.
A parallelagram can be a square, which has four lines of symmetry or a rectangle which has two lines of symmetry but the generic parallelagram has zero lines of symmetry
5 lines
A decagon need not have any lines of symmetry. It can also have 1 or 10 lines of symmetry.