A method of using a mobile device to determine movement, relative to the mobile device, of a print medium configured to be printed on by the mobile device in a print direction, the mobile device including a printer, a first sensor and processing means, the print medium comprising a substrate defining first and second opposite faces, the first face bearing coded data, the method comprising the steps of: using the first sensor to sense at least some of the coded data from the print medium; determining movement of the print medium relative to sensor, based on the coded data sensed by the first sensor.