Nelikulmaisuus

Nelikulmaisuus is a geometric term used to denote the property of having four angles. In standard Euclidean plane geometry, nelikulmaisuus is the defining characteristic of quadrilaterals, i.e., polygons with four sides and four vertices that form four interior angles.

Etymology: The word combines nelikulma (four-angle) with the suffix -isuus, indicating a state or quality.

Definition and basic properties: A figure with nelikulmaisuus is typically a simple quadrilateral, though some sources allow self-intersecting forms. For a simple quadrilateral, the interior angles at the four vertices sum to (4−2)×180 = 360 degrees. Each individual angle is between 0 and 360 degrees, with convex quadrilaterals having all angles less than 180 degrees and concave ones having at least one angle greater than 180 degrees. The term does not specify a particular shape; it covers rectangles, squares, rhombi, trapezoids, and more irregular four-angled figures.

Variants and context: Quadrilaterals are classified by sides (parallelograms, rectangles, squares, rhombi, trapezoids) or by angles (convex, concave). Some geometric results, such as diagonal properties and area formulas, apply specifically to nelikulmaisuus figures.

See also: Quadrilateral, Polygon, Geometry.