The main difference in 4-track cartridge design from 8-tracks is that 4-tracks lack a built-in pinch roller (usually made out of rubber or plastic) which would grip and help move the tape; a hole is left in the cartridge for a pinch roller to be inserted from inside the 4-track player itself. The large opening in the bottom of the 4-track cartridge, for admission of the pinch roller, leaves 4-track tapes more susceptible to trapping dirt and other substances besides those normally found inside cartridges, and requires a greater level of mechanical complexity in the player, as the pinch roller must be inserted and retracted vertically through the bottom of the cartridge.