 Like most monosaccharides with five or more carbons, each aldohexose or 2-ketohexose also exists in one or more cyclic (closed-chain) forms, derived from the open-chain form by an internal rearrangement between the carbonyl group and one of the hydroxyl groups.The reaction turns the =O group into an hydroxyl, and the hydroxyl into an ether bridge (–O–) between the two carbon atoms, thus creating a ring with one oxygen atom and four or five carbons.If the cycle has five carbon atoms (six atoms in total), the closed form is called a pyranose, after the cyclic ether tetrahydropyran, that has the same ring.