The ZX80 only came with 1K of RAM, which had to be shared between program and screen memory, so many customers bought the Sinclair 16K RAM pack, which plugged into the expansion bus (the edge connector at the back, that basically allowed access to the system address and data lines).
The expansion bus was also used for connecting peripherals such as the Sinclair spark printer, and third-party disk drives and joysticks.
How was it possible to use peripherals and the RAM pack at the same time? Did one or the other have a pass-through for the expansion bus?