Machines with the lowest cost and highest reliability are generally those with the fewest mechanical connectors and socketed components, while those with more connectors and sockets are more easily expanded and maintained. One design is to have all circuits on mechanically equivalent boards that plug into a common bus, this is the most modular arrangement and is easily maintained. An alternative design is the single-board computer which places the CPU and a substantial portion of the memory and I/O interface circuits onto a single circuit board. This design is more compact and less expensive.