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Rhombus  plural: rhombi
Definition: A quadrilateral with all four sides equal in length.
Try this Drag the orange dots on each vertex to reshape the rhombus. Notice how the four sides remain the same length and the opposite sides remain parallel.

A rhombus is actually just a special type of parallelogram. Recall that in a parallelogram each pair of opposite sides are equal in length.   With a rhombus, all four sides are the same length.It therefore has all the properties of a parallelogram. See Definition of a parallelogram

Its a bit like a square that can 'lean over' and the interior angles need not be 90°. Sometimes called a 'diamond' or 'lozenge' shape.

Properties of a rhombus

Base Any side can be considered a base. Choose any one you like. If used to calculate the area (see below) the corresponding altitude must be used. In the figure above one of the four possible bases has been chosen.
Altitude The altitude of a rhombus is the perpendicular distance from the base to the opposite side (which may have to be extended). In the figure above, the altitude corresponding to the base CD is shown.
Area There are several ways to find the area of a rhombus. The most common is (base × altitude). Each is described in Area of a rhombus
Perimeter Distance around the rhombus. The sum of its side lengths. See Perimeter of a rhombus
Diagonals Each of the two diagonals is the perpendicular bisector of the other. See Diagonals of a rhombus
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Other polygon topics

General

Types of polygon

Area of various polygon types

Perimeter of various polygon types

Angles associated with polygons

Named polygons