Abstract: Magnetic cards are provided with unique card signatures which include a number of bits, some of which have intermediate magnetic levels. As a result a card reader allocates randomly either a one or a zero to the intermediate level bits. The random allocation provides a test for the genuineness of the card. The intermediate level bits are formed as a result of inherent inconsistencies in the characteristic of a normal magnetic stripe along its length. It is not possible therefore to readily, if at all, reproduce patterns of intermediate level bits in a duplicate because the stripe on the duplicate will have differently arranged inconsistencies along its length.