Heteromecic

Twice a triangular number is a heteromecic number.

[1] Again, then, to take a fresh start, a number is called heteromecic if its representation, when graphically described in a plane, is quadrilateral and quadrangular, to be sure, but the sides are not equal to one another, nor is the length equal to the breadth, but they differ by 1. Examples are 2, 6, 12, 20, 30, 42, and so on, for if one represents them graphically he will always construct them thus: 1 times 2 equals 2, 2 times 3 equals 6, 3 times 4 equals 12, and the succeeding ones similarly, 4 times 5, 5 times 6, 6 times 7, 7 times 8, and thus indefinitely, provided only that one side is greater than the other by 1 and by no other number.