Any number which can be expressed non-trivially as a rectangular array of dots, i.e. excluding the trivial case of a single row. Thus rectangular numbers are non-prime in that they can be expressed as a×b where neither a nor b is 1. For many rectangular numbers, the representation is not unique, so 12 could be drawn as a 4×3 or a 6×2 rectangle. In the case where there is a representation with equal factors it is a square number, so 16 is a square number although it can also be represented as an 8×2 rectangle.