Good point. A similar lack of clarity also tripped me up when some documentation switched from "master key" to "main key" because there actually was an additional concept called a "standard key" which seems to have a lot of potential conceptual overlap with "main key".

https://twitter.com/canadaduane/status/1465737258850344961

Well, there used to be daughterboards (but no sonboards).

There are still other boards you attach to the motherboard. A GPU is a board, a memory module is soldered onto a controller board, etc

"Parentboard" is probably better than "mainboard", but given the choice between "motherboard" and "mainboard", which are the two currently-accepted terms for that particular component, I'm going to go with mainboard.

Edit: isn't the videocard technically a "non-main-board"? I mean nobody called it a "daughterboard" but it kind of technically is one isn't it? And so if "mainboard" implies the existence of "non-main-boards"... that accurately describes its relationship with the video card, which is a baord.