α-glucose and β-glucose are two cyclic hemiacetal forms of glucose which differ only in the configuration of the hydroxyl group (-OH) at the anomeric carbon. Such isomers are called anomers. The six-membered cyclic structure of glucose is called a pyranose structure. Pyranose structure of glucose: The six-membered ring contains an oxygen atom; because of its resemblance with pyran, it is called the pyranose form. α-D-glucose and β-D-glucose are stereoisomers; they differ in the 3-dimensional configuration of atoms/groups at one or more positions. Note that the structures are almost identical, except that in the α form, the -OH group on the far right is down and in the β form, the -OH group on the far right is up.