Kolmnulgiku

Kolmnulgiku is a term found in some Estonian-language geometry and puzzle literature to refer to a three-angled polygon, commonly understood as a triangle. Etymology-wise, it comes from kolm (three) + nurg (angle) + -iku (a diminutive or shape-forming suffix). In formal mathematics, however, the standard term for this shape is kolmnurk, and kolmnulgiku is largely encountered in informal or educational contexts.

A kolmnulgiku is defined by three straight sides and three interior angles; in Euclidean geometry, the sum of these interior angles is 180 degrees. It can be equilateral (three equal sides), isosceles (two equal sides), or scalene (all sides different). When used in tiling or puzzle contexts, kolmnulgiku tiles may refer to triangular shapes that tessellate the plane, often emphasizing the common equilateral triangle in such designs.

History and usage of the term indicate that kolmnulgiku appears mainly in school materials, introductory texts, and puzzle collections from the late 20th century onward as an informal synonym for triangle. It is not widely used in formal mathematical writing, where kolmnurk remains the standard terminology. In design and popular culture, kolmnulgiku may be used to describe triangular motifs or patterns.

Variants and related concepts: educational texts may present phrases such as väike kolmnulgiku (small triangle) or suur kolmnulgiku (large triangle) to indicate scale. Related terms include kolmnurk (triangle) and polügoon (polygon). The concept aligns with the broader study of triangles in geometry, including their properties, classifications, and applications in tiling and design.