Rectangle
(Jump to Area of a Rectangle or Perimeter of a Rectangle)
A rectangle is a foursided flat shape where every angle is a right angle (90°).

means "right angle"  

are equal sides  

are equal sides  
Each internal angle is 90°  
Opposite sides are parallel and of equal length (so it is a Parallelogram). 
Play with a rectangle:
Area of a Rectangle
Area = a × b 
Example: A rectangle is 6 m wide and 3 m high, what is its Area?
Area= 6 m × 3 m
= 18 m^{2}
Perimeter of a Rectangle
The Perimeter is the distance around the edges.
The Perimeter is 2 times (a + b): Perimeter = 2(a+b) 
Example: A rectangle is 12 cm long and 5 cm tall, what is its Perimeter?
Perimeter = 2 × (12 cm + 5 cm)
= 2 × 17 cm
= 34 cm
Diagonals of a Rectangle
A rectangle has two diagonals, they are equal in length and intersect in the middle. 
A diagonal's length is the square root of (a squared + b squared): Diagonal "d" = √(a^{2} + b^{2}) 
Example: A rectangle is 12 cm wide, and 5 cm tall, what is the length of a diagonal?
d = √(12^{2} + 5^{2})
= √(144 + 25)
= √169
= 13 cm
Golden Rectangle
There is also a special rectangle called the Golden Rectangle: