A lock and key system is disclosed including locks of the kind comprising a cylindrical key plug (203) with locking tumblers (205) having a body part (206a) and an integral key sensing part (206b) for engagement with an associated key (100). The key sensing part (206b) of each locking tumbler comprises at least two neighbouring key contacting portions (210a, 211a), confined within a limited region, for possible sliding engagement with a longitudinal code pattern of the associated key, at least one but not necessarily all of these key contacting portions of each locking tumbler actually engaging with a corresponding code portion of the associated key upon fully inserting the latter into the key slot of the lock.