A security turnkey with a dedicated cylinder, comprising a block groove (BN) with a coded block depth (B1, B2, B3), running parallel to the key axis (x), from the key tip to at least the first position (P1) of a coded tumbler row (A2) on the key. The dedicated cylinder has, at least in the rearmost coded position (P1), a tumbler pair corresponding to one of the block grooves (BN) and is provided with a block tumbler (BZ) and an elongated counter block tumbler (BG), whereby the counter block tumbler (BG) stands proud on the cylinder shell (10) when the block groove (BN) is too shallow and thus prevents the complete insertion of a key, by means of the block tumbler pair. At the same time, the block tumbler (BZ) with the counter tumbler (BG) in position (P1), also serve as coding tumblers.; In a lock system employing the above-described security turnkeys for locking installations at least two areas (G1, G2) are defined, whereby in a first area (G1) several additional security elements and a blocking code function and in the second area (G2) a more simple basic coding are foreseen.
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