Here we see a close-up of the BASIC programming language cartridge sitting in the Atari 800's left cartridge slot. The 800 also had a right cartridge slot, but this slot received very little use throughout the system's run. The original purpose of the right cartridge slot was simply to extend the functionality of a cartridge plugged into the left slot. Joe Decuir, one of the 800's designers, says, "We envisioned being able to have a mother cartridge and extensions of that mother cartridge. It never happened."
Only a handful of commercial cartridges--mostly programming utilities--used the right slot, whereas all primary cartridge software used the left slot. Subsequent versions of the Atari 8-bit computer line omitted the right cartridge slot altogether.