An in silico survey of all known 3D-structures of glycoside hydrolases that contain a ligand in the -1 subsite is presented. A recurrent crucial positioning of active site residues indicates a common general strategy for electrostatic stabilisation directed to the carbohydrate's ring-oxygen at the transition state. This is substantially different depending on whether the enzyme's proton donor is syn or anti positioned versus the substrate. A comprehensive list of enzymes belonging to 42 different families is given and selected examples are described. An implication for an early evolution scenario of glycoside hydrolases is discussed.