The two simplest carbohydrates, D-glyceraldehyde and dihydroxyacteone, are also the simplest members of the subclasses of carbohydrates. Glyceraldehyde and carbohydrates containing an aldehyde carbonyl group are aldoses. Those with a ketone group, like dihydroxyacteone are ketoses. The sugars are also known as aldotriose and ketotriose.

Aldotetroses

Two common tetroses (with the same empirical formula), D-erythrose and D-threose, contain two chiral centers, and differ only on the orientation of the hydroxyl group at the C-2 position. This results in four possible stereoisomers, though only the two shown below are found in nature: