I'm guessing this topic has just about run its course, but I thought I would add these (as ever) useful comments from Sim Branaghan, author of the excellent British Film Posters book...
He says that the basic format for British posters has always been the Double Crown (30"x20"), established in this role by the time of our Advertising Stations (Ratings) Act in 1889. Larger posters were thus generally referred to by the number of DCs they contained. Printers created this terminology, so while basic UK formats can be doubled, or quadrupled, or double-quadrupled, after that you have to start referring to number of "sheets" (eg DCs). For example, a pre-war British "six-sheet" would technically be 90"x40" etc. Individual DCs were often referred to by the advertising/publicity men as "Bus Fronts" or "Bus Backs" as this is where they were often displayed. For giant formats, the 60"x40" or "Double Quad Crown" was the biggest individual size standard litho-presses could take, and anything larger had to be printed in sections, often uneconomically trimmed.
The problem, of course, is that this terminology has shifted about. Prior to WW2 derivations of "sheet" always referred to DCs. After about 1950, the phrase was increasingly applied to British-printed export versions of American sizes - so for roughly thirty years or so we had one- three- and six-sheets that matched the US sizes (ie 40"x27", 81"x40" and 81"x80" or thereabouts). But UK three- and six-sheets were defunct by about 1980 - six-sheets disappeared entirely, while three-sheets were replaced by (already existing) 60"x40"s or Double Quad Crowns. The advertising term for these is "four sheet", although printers still called them "Double Quads" (Terry: .. and I suspect that's where the confusion of them sometimes being referred to as 'two-sheets' originated ...) Another name sometimes used was "Bus Stop".... which as we've seen only serves to confuse matters further, since from about 1990 this phrase has been increasingly applied to the new size of 72"x48".
The easiest way to avoid confusion is simply to describe the poster in terms of its dimensions. But even THIS can be fraught with difficulty, since you then has to decide which dimension (height/width) to give first. And again, there has always been a split - printers always quote height/width, but the Advertising industry has always quoted width/height. If you look at old (50s/60s) pressbooks, quads are quite often listed as "40x30"......
Obviously, there's some duplication of the information which has already been laid out earlier in the thread, but i thought I would add Sim's comments anyway.
Best wishes,
Terry -
www.quadbod.co.uk