A method of magnetic encoding of credit instruments having a strip form magnetic recording medium provides at least one data field on the strip. First and second magnetic field orientations are selected for recordation on the medium of successive, adjacent bit regions in an alternating field orientation pattern with the transition between such orientations defining the transition from one bit region to the next, signalling the initiation of a recorded bit representing either a binary "1" or a binary "0". A first bit region length d.sub.1 for representing one of the binary values "0" and a second bit region length d.sub.2 for representing the other binary value as well as the ratio of d.sub.1 /d.sub.2 are selected, with d.sub.1 /d.sub.2 being from about 0.1 to about 0.5. Thus, the magnetic field transition between adjacent bit regions signals a binary bit and the length of each bit region represents its binary value. Data is recorded in binary format on the medium in one or more data fields using the bit length d.sub.1 and d.sub.2 and ratio d.sub.1 /d.sub.2 so selected. The length of the data fields depends upon the actual value of the data so recorded. The variable data field length assures against jackpotting by unauthorized masking of one of multiple data fields.