Numbers of the form n(n + 1)/2 are called triangular numbers. This is because and the left-hand side of this equation can be represented by a triangle as in Fig. (a). Numbers of the form n are called square numbers, or simply squares, for obvious similar reasons. We can also have rectangular numbers, but there is little point in defining them as numbers of the form m . n, since then they will include all the numbers. Instead we define a rectangular number as one which is of the form n(n + 1) —that is, the rectangle concerned must have one side exceeding the other by exactly one unit.