Most of what has been eaten is diegested in the duodenum.The duodenum is divided into four sections. The first three sections form a "C" shape - the first part of the duodenum comes off the pylorus and makes a sharp ~90 degree turn to become the second part of the duodenum. The second part of the duodenum is where the pancreatic duct (and sometimes, separately, the common bile duct) empty into the gastrointestinal tract. The duodenum makes another sharp ~90 degree turn into the third part of the duodenum before joining with the jejunum. The duodenum is a retroperitoneal structure.
The name duodenum is from the Latin duodenum digitorum, twelve fingers' breadths or inches.