sidejo

Decode it and it explains itself. Sidi at rank 214 is to sit. -Ej- is the place suffix, the one that makes lernejo a school out of lerni and a necesejo a lavatory out of necesa. So a sidejo is a sitting-place: where an organisation sits. English made the identical metaphor and mostly forgot it, which is why the seat of government sounds like furniture until you stop and look at it.

It is not a chair. This is the trap. A thing you sit on is a seĝo, and a seat in a theatre or a train is a sidloko. Sidejo is the institutional sense only: the registered address, the building the organisation is run from. Nor is it quite an oficejo at 948, which is any office at all - a large body may have twenty oficejoj and exactly one sidejo.

The one every Esperantist knows. The Universala Esperanto-Asocio has had its sidejo at Nieuwe Binnenweg 176 in Rotterdam since 1955, and the movement talks about it the way English football talks about a club's ground: la Centra Oficejo, the CO, is where the archive and the library and the book service live. Before Rotterdam it was Geneva, and before that London. For a movado that exists mostly in the post and now mostly online, having one physical address at all is a quietly important fact about it.