In the original incarnation of the Annex Project, the mainframe-tans were all literally giantesses, over 50 feet tall. This has mainly fallen out of fanon, but as an allusion to it, the mainframe-tans are shrouded in myth to all the other OS-tans, including mythical tales of mysterious mainframe-tans being giantesses.
- IAS (aka "Institute for Advanced Study Machine")
- MCP (aka "Master Control Program")
- NOS (aka "Network Operating System)
- NTT DIPS
- RCA TSOS (aka "VS/9", "BS2000")
- SCOPE (aka COS, Chippewa Operating System)
- VSE (fka "DOS/360")
The first OS-tans and computer-tans were mainframes, but The Guild as it is known today didn't originally exist. The earliest mainframes, created in the 40's and 50's largely lived in isolation from one another. They were created with super abilities, and were in a sense goddesses and had many servants and caregivers, but mainly served their people on a local scale, and were generally down-to-earth.
In the 60's, values changed and a new generation of Mainframe-tans were born. This generation were even more powerful than their predecessors, were in clans competing for power, and served on a large scale. While the early mainframe-tans mainly lived in obscurity, this generation was a lot more well-known, and their fame fostered belligerent, stuffy attitudes, and a techno-aristocracy. Some hobbyist computer-tans existed during this time, but weren't a threat, but the minicomputer-tans were starting to.
In the face of competition against the microcomputers, the mainframes (begrudgingly) established The Guild together. However, there was a schism between the mainframes, and only some of the mainframes joined, and these tended to be the most arrogant of them, and its creation did not deter the mainframe clans from fighting each other.
The mainframe-tans were all created powerful, but very few were born and raised with their full, goddess powers; nor born into The Guild. Their position in The Mainframe Guild isn't life-long either, and anyone has the right to resign from their position to retire back to a normal life, though naturally they are stripped of much of their power and memory of sensitive information.
In fact, there are few permanent members, with more and more questioning the point of being in the Guild anymore when it's mainly a skeleton crew without the prestige it used to have. But some still stay for the sake of tradition and keeping up the Mainframe mythos.