In June, 1927, someone had a brilliant idea. Or, at least, that’s when the idea was first codified, at a meeting of the Royal Institute of International Affairs at Chatham House in London. All attendees of the meeting could quote comments made at the meeting, but they weren’t allowed to say who had made the comment.

This became known as the Chatham House Rule, and the most recent incarnation is defined thus:

“When a meeting, or part thereof, is held under the Chatham House Rule, participants are free to use the information received, but neither the identity nor the affiliation of the speaker(s), nor that of any other participant, may be revealed.”