Ristküljel

Ristküljel is the Estonian term used in geometry to denote the rectangle, a four‑sided plane figure defined by four right angles. In standard Estonian geometry vocabulary, the shape is usually called ristkülik, while ristküljel appears in grammatical contexts such as attributive or locative phrases (for example, “ristküljel pindala” meaning the area of a rectangle).

In Euclidean geometry, a rectangle is a parallelogram with four right angles. Opposite sides are equal and parallel, and the diagonals are equal in length and bisect each other. If the side lengths are a and b, the area is A = a × b and the perimeter is P = 2(a + b). The diagonal length is d = sqrt(a^2 + b^2). A square is a special rectangle with a = b.

In Estonian usage, common phrases include ristküliku pindala (area of a rectangle), ristküliku ümbermõõt (perimeter of a rectangle), and ristküliku diagonaal (rectangle diagonal). Rectangular shapes are widely used in design, architecture, tiling, and computer graphics due to their simple properties and predictable measurements.

The rectangle is one of the basic quadrilaterals in geometry and serves as a fundamental example of a parallelogram with right angles. Its properties—right angles, equal diagonals, and straightforward area and perimeter formulas—make it a foundational concept in both mathematical theory and practical applications. In Estonian mathematical language, these ideas are integrated into standard terminology and teaching, with phrases that routinely combine ristküliku with measurements and operations.