Rhombus

In geometry, a rhombus or rhomb is a quadrilateral whose four sides all have the same length. The rhombus is often called a diamond, after the diamonds suit in playing cards, or a lozenge, though the latter sometimes refers specifically to a rhombus with a 45&deg; angle.

In general, a polygon whose sides have the same length is called equilateral, so a rhombus is an equilateral quadrilateral. Every rhombus is a parallelogram, and a rhombus with right angles is a square. (Euclid's original definition of rhombus excluded squares, but modern mathematicians prefer the inclusive definition.).

The english word &ldquo;rhombus&rdquo; derives from the Ancient Greek ῥόμβος (rhombos), meaning “spinning top”. The plural of rhombus can be either rhombi or rhombuses.

Properties
Every rhombus has two diagonals connecting opposite pairs of vertices. Using congruent triangles, one can prove that the rhombus is symmetric across each of these diagonals. It follows that any rhombus has the following two properties: The first property implies that every rhombus is a parallelogram. A rhombus therefore has all of the properties of a parallelogram: opposite sides are parallel, adjacent angles are supplementary, and the two diagonals bisect one another.
 * 1) Opposite angles of a rhombus have equal measure.
 * 2) The two diagonals of a rhombus are perpendicular.

Not every parallelogram is a rhombus, though any parallelogram with perpendicular diagonals (the second property) is a rhombus. In general, any quadrilateral whose two diagonals are perpendicular is called a kite. Every rhombus is a kite, and any quadrilateral that is both a kite and parallelogram is a rhombus.

Origin
The word rhombus is from the Greek word for something that spins. Euclid used ρόμβος (rhombos), from the verb ρέμβω (rhembo), meaning "to turn round and round". Archimedes used the term "solid rhombus" for two right circular cones sharing a common base.

Rhombus in mathematics

 * The dual polygon of a rhombus is a rectangle.
 * One of the five 2D lattice types is the rhombic lattice, also called centered rectangular lattice.
 * Three-dimensional analogues of a rhombus include the bipyramid and the bicone.