neljakandiliste

Neljakandiliste is a term used in geometry to refer to quadrilaterals, polygons with four sides and four interior angles. The name is derived from the idea of four angles, and in many Estonian-language texts it serves as a standard descriptor for four‑sided figures. A neljakandiliste is typically denoted by its vertices in order, for example ABCD, with the interior angles summing to 360 degrees.

Classification and examples: Neljakandiliste can be classified as convex or concave depending on whether all interior angles lie inside the figure. They also include common special cases such as rectangles (all angles are right angles), parallelograms (opposite sides are parallel), rhombi (all sides equal), and squares (both a rectangle and a rhombus). A trapezoid (or trapezium) has at least one pair of parallel opposite sides. These categories are not mutually exclusive; for instance, a square is a particular kind of rectangle, rhombus, and parallelogram.

Properties: In a neljakandiliste, the diagonals generally intersect and can have varying lengths. In a parallelogram, diagonals bisect each other. In a cyclic neljakandiliste (one inscribed in a circle), opposite angles sum to 180 degrees. The area can be computed by splitting the figure into two triangles or by using formulas that involve the lengths of sides and the angle between them; for a general quadrilateral, Bretschneider’s formula provides a comprehensive expression when the angle between the diagonals is known.

Applications and usage: Neljakandiliste are fundamental in architecture, land surveying, computer graphics, and design, where understanding four-sided shapes supports tasks from layout planning to structural analysis. The term is common in Estonian mathematical discourse, while in English the standard term is quadrilateral. See also quadrilateral, polygon, geometry.