notation ?^x,y,... is traditionally used to specify a fatty acid with double bonds at positions x,y,.... (The capital Greek letter "?" (delta) corresponds to Roman "D", for Double bond). Thus, for example, the 20-carbon arachidonic acid is ?^5,8,11,14, meaning that it has double bonds between carbons 5 and 6, 8 and 9, 11 and 12, and 14 and 15.

In the context of human diet and fat metabolism,