It means 4 lines of symmetry as for example a square has 4 lines of symmetry
This is a little bit like asking why a square has four sides. A regular pentagon is defined in such a way as to require that it has 5-fold rotational symmetry.
It has rotational symmetry of order 2.
Yes, it does.
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Parallelograms: 2-fold Square: 4-fold n-fold symmetries refer to rotational symmetries. Consequently, any symmetries about axes that these and other quadrilaterals may have are not relevant to this question.
It can be rotated 4 different places and still maintain all of its characteristics when a design has four fold symmetry.
This is a little bit like asking why a square has four sides. A regular pentagon is defined in such a way as to require that it has 5-fold rotational symmetry.
Because it only has four sides. So it can only have two lines of symmetry because if you fold it the sides will be the same shape.
A two-fold symmetry has a 360 degrees rotation. A three-fold rotational symmetry, on the other hand, has 120 degrees, and on a horizontal axis, a symmetry has 180 degrees.
It has rotational symmetry of order 2.
4
two-fold
Yes, it does.
line
Yes, it does.
A line has rotational symmetry of order 2.
Sting rays have a two fold axis of symmetry which would make it closed symmetry.