The square-based pyramid, a shape which usually has four ... with a result that a noticeable depression runs down the middle of each face; a peculiarity shared, as far as is known, by no other ...

Right, what about triangles? Triangles become pyramids. You're now a square based pyramid with 4 triangular faces and a square face for your bottom. You can get pyramids with different shaped bases.

"The Great Pyramid at Giza is known to have an amazing character of concavity that each of its four faces is slightly ...