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A rectangle is a geometric shape with four sides where all angles are 90°. If all the sides of a rectangle have the same length it's usually called a square instead.

The word *rectangle* comes from the Latin word *rectangulus*, a combination of *rectus* (right) and *angulus* (angle), which refers to the rectangle's four right angles (90°).

Rectangles are often depicted with the longest sides horizontally and the shorter sides vertically. The distance between the vertical sides are often called the *base*, while the distance between the horizontal sides are called the *height*.

When calculating a rectangle's diagonal, perimeter and area it does not matter which side is the base and which side is the height. The tool therefore uses the more neutral naming, a and b. If neither of them are specified there can often be multiple solutions. For example, a rectangle with perimeter 10 and area 6 has the sides 2 and 3, but it is unclear if it is a that is 2 and b that is 3 or if it's the other way around. In these situations the tool will just show one the two possible solutions.

The area of a rectangle can easily be calculated by multiplying the length of side a and b. To get the perimeter the sum of a and b is multiplied by 2 (because the opposite sides are identical). The diagonal can be calculated using the Pythagorean theorem.

Diagonal = √ a² + b²

Perimeter = 2a + 2b

Area = a×b

Perimeter = 2a + 2b

Area = a×b