For a shape to be a rhombus, it has to have four straight sides of equal length and the opposite angles need to be the same. The most common rhombus shape is a diamond. However, technically, a square can also be classified as a rhombus because it has four sides of equal length and the opposite angles are equal. However, it is important to note that not all rhombuses are squares because rhombuses don’t all have 90° angles.