Abstract: A credit card imprinter is disclosed herein in which printing of the sheet is inhibited or prevented if the credit card is inadvertently incorrectly positioned on its support. The imprinter has a manually-operated printing carriage movable along a support member on which a sheet can be arranged to overlie an embossed credit card. In one embodiment a combined detent and trip mechanism is provided to prevent movement of the printing carriage over the support member and to raise a printing roller in the carriage away from the support member. The mechanism is disabled by the correct insertion of the credit card. In another embodiment, a resilient means is provided for preventing the sheet from being positioned on the support unless the credit card has been properly positioned thereon.