A rhombus (plural rhombi) is a four-sided polygon in which every side has the same length. In any rhombus, opposite sides are parallel. Therefore, the rhombus is a special case of the parallelogram.
The area of any rhombus is a half the product of the lengths of its diagonals:
A = ½ (D1 x D2)
Because the rhombus is a parallelogram with four equal sides, the area also equals the length of a side (B) multiplied by the perpendicular distance between two opposite sides (H):
A= B x H