Call Stores were the system's read/write memory, containing the data for calls in progress and other temporary data. They had a 24-bit word, of which one bit was for parity check. They operated similar to magnetic core memory, except that the ferrite was in sheets with a hole for each bit, and the coincident current address and readout wires passed through that hole. The first Call Stores held 8 Kilowords, in a frame approximately a meter wide and two meters tall.