Abstract: A magnetically encoded card (2) has a recording track (6, 8) containing data (10, 14, 16, 18) which can be read and re-written by a card reader (30), and a further recording (12) at a much higher spatial bit denisty than the data (10, 14, 16, 18). The high denisty recording (12) is formed using a special write head capable of operating at greater bit densities than the head of the card reader used to write information to the card. The card (2) is deemed valid only if the presence of the high density recording (12) is detected by a read head (34) of the card reader.