Perusreunan

Perusreunan is a Finnish geometric term that designates the base edge of a solid, particularly the edge that forms the boundary of the base face of a prism or pyramid. The word is composed of perus, meaning base, and reuna, meaning edge. In common use, perusreunan refers to any edge belonging to the base polygon of a three‑dimensional object, and the genitive form perusreunan is used when describing properties of that edge, such as perusreunan pituus (the length of the base edge) or perusreunan suunta (the edge’s orientation).

In elementary and secondary mathematics, perusreunan serves to distinguish base edges from lateral edges, which connect the base to the top face. For a square pyramid, the base is a square, and the four perusreunat constitute the edges of that square base. Similarly, in a triangular prism, the base consists of two parallel triangles, and the perusreunat are the corresponding edges of those base triangles.

Beyond pure geometry, the concept appears in technical drawing and computer-aided design when describing footprints or baselines of extruded shapes. The perusreunan helps define how a two-dimensional base shape is extended into three dimensions and how lateral faces are attached.

See also: reuna (edge), perus (base), geometria (geometry).