Nietcirkels

Nietcirkels is a term found in some Dutch-language sources to denote shapes that are not circles. In strict geometry, a circle is the set of all points at a fixed distance (the radius) from a fixed center. Nietcirkels encompasses any plane figure that does not meet this criterion, including ellipses, polygons with rounded corners, and irregular curves. The term is not a standard mathematical classification, but a descriptive label used to contrast true circles with other roundish shapes.

Mathematically, circles have constant curvature, equal to 1 over the radius. Nietcirkels, by contrast, typically have varying curvature along their boundary. Examples range from ellipses, which have two principal axes and no fixed distance from a single center, to more complex curves or shapes with nonuniform distance from any central point. Some non-circles—such as certain curves of constant width—can resemble a circle closely without being one.

Usage and applications vary. In geometry education, nietcirkels help illustrate the uniqueness of the circle among closed curves. In design and visual arts, the term may describe shapes that suggest circular form while permitting intentional imperfection for texture or interest. In computer vision or pattern recognition, distinguishing circles from nietcirkels can be a practical task for shape detection and classification.

Etymology: nietcirkels combines niet- (not) with cirkel (circle) in Dutch. The term appears in informal discussions and teaching materials rather than as a formal object in mathematics. See also circle, ellipse, and curvature for related concepts.