Since the cube root of 231 is between 6 and 7 inches, how do they actually measure a gallon? (No, I'm not a math genius, just checked it.) Is it x by y by z inches, where x ≠ y ≠ z?

Jack and Pottle are new ones for me. Is that really a traditional measure, or did someone more recently make it up so that everything would be 2 of something else?