Abstract: A novel lock system utilizes a cylinder lock and a companion key with a projection extending from one side thereof to improve security. The cylinder lock comprises a shell, a core which is rotatably mounted within the shell and a plurality of pin tumbler stacks normally prevent movement between the shell member and the core. Shear lines are defined between the shell and the core, and also within each tumbler stack. The core also includes an outer surface, a longitudinal keyway and a cut-out which is normally offset from, but alignable with, at least one of the tumbler stacks. A plate member is disposed within the cut-out for movement between a first position, wherein the plate member acts as an extension of the core surface to prevent a tumbler stack from entering the cut-out and a second position, wherein the plate member does not prevent a tumbler stack from entering the cut-out.