DUPLÉ, among mathematicians, denotes the ratio of 2 to 1. Thus the ratio of 8 to 4 is double, or as 2 to 1.

Sub-duple ratio is just the reverse of the former, or as 1 to 2. Thus the ratio of 4 to 8, or 6 to 12, is sub-duple, or as 1 to 2.