Founded by refugees from HP in the mid-1970s. Famous for the NonStop line of fault-tolerant computers. Every part of the machine - disk, power supply, bus, processors, you name it - was hot swappable. If any part failed you just yanked it out and dropped in a replacement, all with the machine online. The machines had multiple processors, typically 4, working in (oddly enough) tandem. A process would run 'live' on one processor but keep a backup of itself which could take over seamlessly if the first processor failed. This backup process could be on another processor of the same machine, but was more frequently on another remote machine. The backup methodology was called checkpointing.

These machines were used by banks and other financial institutions for processing critical transaction-based data. The Canadian banks headquartered in Toronto (all within blocks of each other) all had primary Tandem machines in their subterranean datacenters, and the remote paired machines lived in their secret twin data centres in Markham - all, also, within blocks of each other.

I used the Tandem TNS-II in the mid-80s, programming in TAL to the original Guardian OS. It was a lot of fun! Screens were written in SNOBOL which was less fun, but I was writing the back end so that was somebody else's problem.