The duodenum has four parts, named first through fourth.
The first part of the duodenum, aka superior, extends to the right as a continuation from the pyloric canal. On radiological examination, this part of the duodenum is seen to have a smooth wall and is named "duodenal cap". Anteriorly, it is in contact with the gall bladder. The dorsal mesentery of the first part of the duodenum contributes to the greater omentum.
The second part of the duodenum, aka descending, is located on the right side of L1 to L3 vertebrae. It has the common bile duct and the main pancreatic duct opening into it together at the major duodenal papilla. A minor duodenal papilla may be located more superiorly, marking the opening of an accessory pancreatic duct. The second part's anterior surface is related to the liver and to loops of small intestine, while its posterior surface rests on the right kidney. The dorsal mesentery (along with that of the third part) is "fixed" due to fusion of the mesentery with paritetal peritoneum.
The third part, aka horizontal, crosses the L3 vertebra from the right to the left, anterior to the inferior vena cava and aorta Here, the dorsal mesentery (along with that of the second part) is "fixed" due to fusion of the mesentery with paritetal peritoneum.
The fourth part, aka ascending, is short, and turns forward and becomes continuous with the jejunum at the duodenojejunal flexure, which is suspended from the posterior abdominal wall by a fibromuscular band known as the suspensory ligament of the duodenum (ligament of Treitz). The fourth part retains a mesentery, and so is mobile.