The triangle is the only polygon for which the shape remains the same, as long as the length of the sides doesn't change (a rafter).
The triangle is a simple figure, but however, many triangles can be distinguished, many of them carrying an own name. For the acute-angled triangle (or acute triangle) all angles are smaller than 90 degrees. When a triangle has an obtuse angle, it's an obtuse-angled triangle (or obtuse triangle).
