A lock comprising: a shell having a bore formed therein; a cylindrical plug having an axis and an axially-extending keyway adapted to receive a conforming key (200, 220), wherein the bore formed in said shell is adapted to receive the cylindrical plug; a plurality of tumbler pin assemblies (50, 58) disposed within tumbler pin holes (13, 23) formed in said shell and said cylindrical plug and adapted to control rotation of the cylindrical plug with respect to said shell, each tumbler pin assembly (50, 58) being reciprocally moveable within an associated tumbler pin hole (13, 23), wherein engagement of the tumbler pin assemblies (50, 58) by the biting (210, 230) of a properly configured key (200, 220) inserted into the keyway will position the tumbler pin assemblies (50, 58) within their associated tumbler pin holes (13, 23) so as to permit the cylindrical plug to rotate with respect to said shell; an axially-oriented sidebar coupled with the cylindrical plug and having an outer end that is engageable with a sidebar groove formed in the wall of the bore formed in said shell, wherein engagement of the sidebar with the sidebar groove blocks rotation of the cylindrical plug with respect to the shell; and a slider disposed with respect to the cylindrical plug and adapted to move axially relative to the cylindrical plug, when engaged by a properly configured key (200, 220) inserted into the keyway, from a first position to a second position, whereby in the first position, the slider interferes with the sidebar to block the sidebar from moving radially inwardly toward the axis of the cylindrical plug, and when the slider is in the second position, the slider does not interfere with the sidebar, thereby permitting the sidebar to move radially toward the axis of the cylindrical plug so that the sidebar disengages from the sidebar groove and permits the cylindrical plug to rotate within the bore formed in the shell; wherein the lock is unlocked when the tumbler pin assemblies (50, 58) are positioned so as to permit the cylinder plug to rotate with respect to the shell and the slider is moved to its second position so as to permit the sidebar to move radially inwardly and disengage from the sidebar groove. a shell having a bore formed therein; a cylindrical plug having an axis and an axially-extending keyway adapted to receive a conforming key (200, 220), wherein the bore formed in said shell is adapted to receive the cylindrical plug; a plurality of tumbler pin assemblies (50, 58) disposed within tumbler pin holes (13, 23) formed in said shell and said cylindrical plug and adapted to control rotation of the cylindrical plug with respect to said shell, each tumbler pin assembly (50, 58) being reciprocally moveable within an associated tumbler pin hole (13, 23), wherein engagement of the tumbler pin assemblies (50, 58) by the biting (210, 230) of a properly configured key (200, 220) inserted into the keyway will position the tumbler pin assemblies (50, 58) within their associated tumbler pin holes (13, 23) so as to permit the cylindrical plug to rotate with respect to said shell; an axially-oriented sidebar coupled with the cylindrical plug and having an outer end that is engageable with a sidebar groove formed in the wall of the bore formed in said shell, wherein engagement of the sidebar with the sidebar groove blocks rotation of the cylindrical plug with respect to the shell; and a slider disposed with respect to the cylindrical plug and adapted to move axially relative to the cylindrical plug, when engaged by a properly configured key (200, 220) inserted into the keyway, from a first position to a second position, whereby in the first position, the slider interferes with the sidebar to block the sidebar from moving radially inwardly toward the axis of the cylindrical plug, and when the slider is in the second position, the slider does not interfere with the sidebar, thereby permitting the sidebar to move radially toward the axis of the cylindrical plug so that the sidebar disengages from the sidebar groove and permits the cylindrical plug to rotate within the bore formed in the shell; wherein the lock is unlocked when the tumbler pin assemblies (50, 58) are positioned so as to permit the cylinder plug to rotate with respect to the shell and the slider is moved to its second position so as to permit the sidebar to move radially inwardly and disengage from the sidebar groove.