The Electron's sideways address space was exposed only by the addition of a Plus 1 add-on or a third-party equivalent; the Plus 1 also introduced cartridge slots that were carried over into the BBC Master design as an alternative way to package ROMs.
The Electron's horizontal address space was revealed only when a Plus 1 add-on or a third-party substitute was installed, and this add-on also brought about the introduction of cartridge slots in the BBC Master model, which served as an additional way to package ROMs.