A key with one or more movable elements that slide within the blade of a substantially flat key for operating a lock includes a blade configured to be inserted into a keyway of a lock and including contour features formed on an outer surface of the blade that mate with corresponding contour features of the keyway. The key includes at least one movable element disposed within a recess formed in the blade and including a contour surface exposed to the exterior of the key blade. The contour surface may include external contour features that correspond in shape to at least a portion of the contour features of the keyway. The movable element may be movable within the recess, when it engages a blocking member within the keyway, and after such movement, the element will engage a locking element within the lock.