Nothing about a flat Letter flyer is interesting except what you put on it. That's the entire job, and it's why this is the one that comes back with notes.
There's no fold to open. No die cut doing half the talking. No unusual shape to hide a weak idea behind. One rectangle, one side facing up, and whatever you decided at four in the afternoon. Most formats hand you something to lean on. This one hands you a margin.
It's squarer than people expect, too, and that catches you in small ways. Two columns come out broader than you drew them. A layout that felt tall in your head lands wide on the page. It doesn't fight you — it just rewards horizontal thinking and quietly punishes anything dragged in from a taller format.
Flat Letter is what the American office runs on: price lists, class handouts, event programmes, real estate listings, the thing taped inside a shop window at eye height. US printers call the flat version a flyer and save brochure for anything folded. In the UK the same piece of paper is a leaflet, and nobody is strict about it. Handbill is the older word, and it still turns up in music printing.