The side is called the 'Striking Surface' .
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A matchbox is rectangular in shape. A rectangle has 2 lines of rotational symmetery. So, a matchbox has 2 lines of rotational symmetry.
The flat side of a three dimensional solid is called the face. The total area of all of the faces is called the surface area.
A triangle has three sides. The two sides that are perpendicular are called legs and the diagonal side is called the hypotenuse. Whichever side is horizontal (usually one of the legs) is also called the base.
The same as for any other three dimensional rectangular object: Length X Width X Depth.
That number is called the "perimeter" of the triangle.