Abstract: A method of identifying a data carrier such as a credit card, benefit card, access control card, identity card or stored value card, carrying data in the form of a plurality of rational number in a sequence, successive numbers having a marker between them, comprises a) measuring the distance from a fixed point on the data carrier to a given part of a marker, b) dividing this distance by the distance between corresponding give parts of successive markers, thereby obtaining a rational number, and c) subtracting this rational number from the number in the sequence adjacent the marker in a given direction, thereby obtaining a further rational number which is capable of identifying the data carrier uniquely.