A document such as a credit card formed with magnetizable indicia in the form of short and long individual bars intermingled according to a code to produce, when read by a system having a magnetic reading head, an electrical representation of the indicia to enable automatic identification of the card. The document can be passed beneath the reading head over a wide range of speeds thereby enabling the use of an inexpensive reading system. When the document is a plural layer credit card, the code is formed on an inner surface of the card to prevent erosion and alteration of the code.