The duodenum has four parts; superior, descending, inferior and ascending part. Together these parts form a 'C' shape, that is usually around 25cm long, and wraps around the head of the pancreas

To rememner the length of these parts use the mnemonic;

"Counting 1 to 4 but staggered":
1st part: 2 inches
2nd part: 3 inches
3rd part: 4 inches
4th part: 1 inch