A scan cell (S-cell) is a basic storage unit for a scan-based sequential circuit. Depending on different requirements on the circuit under test, many design variations of S-cells exist. In this paper a systematic method to classify all practical S-cells is presented. Based on a novel graph representation and a set of design criteria, we find that all practical S-cells can be classified into 19 schematic structures which can be further divided into 66 different substructures. Among all of these substructures less than 10 have been previously identified and used.