One of the most versatile features of the BBC micro in its many forms was the ability to plug in additional ROMs containing all manner of filing systems, programming languages and utilities. The limit to this was the space inside the BBC for ROMs. This was solved on the BBC Model B by third-party ROM expansion boards which would allow the maximum number of ROMs to be available at once.
When the Master Series was introduced, Acorn came up with an ingenious built-in solution: the system of ROM cartridges. Two ROM cartridge sockets were provided on the Master 128, each one capable of taking two ROMs, with these four ROMs having a reserved place in the BBC ROM table at ROMs 0 to 3.
In addition to the official Acorn ROM cartridges, third party manufacturers like Watford Electronics, Care Electronics and Peartree produced their own cartridges. I hope you will click on the picture links above to learn more about some of these Master ROM cartridges.