They are plane figures
They are closed
They are bounded by 3 or more straight sides
They have the same number of sides as vertices
Their exterior angles add to 360 degrees
Their interior angles add to (n-2)*180 degrees, where n is the number of sides
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irregular polygons don;t have equal angles
That they all have the same shape and sometimes size
It's quite acceptable to call them 20-29agons but what's more important is knowing how to work out their properties
Basically it is working out the properties of triangles and angles of various polygons.
Polygons that fit together with no gaps or overlaps are said to tessellate with each other.