Disaccharides:

The ketone or aldehyde set on the anomeric carbon atom of one monosaccha- ride can react with the hydroxyl group of a second monosaccharide to form a disaccharide.  The covalent bond established is known as a glycosidic bond.  Lactose is a disaccharide   formed among the anomeric carbon (C-1) of D- galactose and C-4 of D-glucose.  Because the anomeric carbon of the galactose molecule is included in the bond and is in the    configuration, this is known as a   (1→4) bond that can be abbreviated as   1-4.  Maltose is a disaccharide formed among the C-4 and C-1 positions of two glucose units. Moreover, here the configuration of the anomeric carbon atom included is the    form and thus the bond is known as an   (1→4)   bond or abbreviated as   1-4.  Intended for lactose and maltose, one of the anomeric carbons has been used to form the bond leaving the second anomeric carbon free. Therefore both lactose and maltose have a reducing end.  In compare, sucrose is a disaccharide formed through bond formation among the anomeric C-1 of glucose and the anomeric C-2 of fructose so which sucrose lacks a free reducing group.

Figure:   Structure of common disaccharides