At a guess you're talking about road signs.
Rectangular signs give information
Triangular signs give warnings
Circular signs give orders
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You can have a triangular prism but not a triangular cuboid which is a rectangular shape.
Pyramids only have square bases.Triangular prisms have triangle bases.Pyramids have triangle "faces".Triangular prisms have rectangular "faces".
One difference: An octahedron has 8 faces, a triangular prism has 5. An octahedron (="eight faces") is a regular high-symmetry solid with all the faces identical. A triangular prism has a triangular base, a triangular roof, and 3 rectangular sides (walls).
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Unlike a tetrahedron (a four-faced shape with all triangular faces), a pyramid has a square (or rhomboid) base and triangular sides. Thus, a view from any side will yield a triangular appearance, while a view from below or above shows a square or rectangular shape.