exit 44 which has a shape that facilitates coupling to keyed media entrance 26 of print head module 12 first keyed protrusion 46 includes surface 50 which has similar dimensions to first key aperture 32 second keyed protrusion 48 includes surface 52 which has similar dimensions to second key aperture 34 dimensions of first key aperture 32 second key aperture 34 first key protrusion 36 and second key protrusion 38 are arbitrary but only a media cartridge 14 with a corresponding keyed media exit 44 can couple to print head module 12 certain media such as tickets may require security to control their usage for this purpose media cartridge 14 may include an electronic lock 42 that may only be unlocked through unlock circuitry 43 by an authorized user of media 30 and computer 10 for this purpose keyed media exit 44 may electrically couple to keyed media entrance 26 the password to be entered at computer 10 may be encrypted such as by public key encryption with a public key registered to the user with reference to fig2 keyed media entrance 26 of print head module 12 is illustrated in detail the width of media entrance aperture 40 corresponds to the width of media 30 with reference to fig3 keyed media exit 44 of media cartridge 14 is illustrated in detail when engaged keyed media entrance 26 of print head module 12 and keyed media exit 44 of media cartridge 14 are held together by friction first keyed protrusion.