prism
A polyhedron with parallel ends of the same polygonal shape and size, the other faces being parallelogram-shaped sides.
prism: a solid with two matching parallel faces and flat sides
A prism is a three-dimensional solid with two identical polygonal faces at opposite ends (called bases) and rectangular or parallelogram faces connecting them. In construction, the term most commonly refers to triangular prisms and rectangular prisms, though pentagonal and hexagonal prisms appear in specialized applications. The bases are always parallel to each other and the same size and shape, which is what defines the geometry.
Triangular prisms are the most visible prisms on a building site. Roof trusses stand on triangular bases. Concrete or stone wedges used for shimming, leveling, or creating slopes follow this geometry. A right-angled prism has perpendicular sides; an oblique prism has slanted sides but maintains the same base geometry. The angle and length of the connecting faces depend on the specific application, but the parallel-bases rule always holds.
Dimensional and material reality
Concrete prisms are cast as test specimens measuring 100 mm by 100 mm by 200 mm in compression testing. Structural concrete roofing elements, called prism tiles or prism blocks, use the shape to interlock and shed water. Granite and slate cladding may be ordered in prism form for decorative corner treatments. Foam or mineral wool insulation blocks are routinely cut into prisms for fitting around irregular frames and penetrations.
The term appears in optics and surveying, where a prism is a transparent optical tool that bends light; construction workers using laser levels or transit instruments work with glass prisms inside those devices. But on the building surface itself, prism refers to the solid form. Problems arise when bases are not truly parallel, when materials warp, or when load-bearing prisms are installed at incorrect angles. A shimming prism placed at an angle opposite to what the structure needs will create stress concentration rather than distribute load.
The name comes from Greek and Latin roots meaning something cut or sawn. It is fundamentally a geometric term that the building trades adopted because the shape is easy to cast, cut, and understand. When a specification calls for a prism, it is specifying geometry, not material or finish, though context always determines what that prism is made from.