U = Unimpeachable. The directors have gone to the most extreme lengths to ensure the detailed and accurate representation of the period. They may precede the film's release with a lecture tour advising people not to go and see it, as it is a realistic portrayal of The Past and therefore just too damn harrowing. They may insist that cinema proprietors help them to maintain the illusion of actual time-travel, for example by infecting half the audience with smallpox or spraying them with machine-gun fire as they step into the aisle. Like The Past, these films are usually rather long.

PG = Pretty Good. A large amount of historical research has clearly gone on, and the film does not wear its erudition lightly; on a very few, minor occasions, professional historians of the era might have occasion to snigger to themselves, but they are probably bluffing. There are plenty of references to child poverty and syphillis, and a great deal of on-screen mud. If the film is not American, the actors will have been provided with gallons of Panda Pop in order to accelerate the appearance of genuinely bad teeth.

NQP = Not Quite Perfect. By this stage, the film's producers may have regrettably slid into the regrettable habit of trying to make The Past watchable. A few cute urchins and perhaps some veiled allusions to the lack of sanitary facilities are the limit of social reconstruction. Errors are spottable by history graduates and bright A'Level students. Improbable nudity and/or wet t-shirt scenes begin to appear.

SI = Stretching It. Everybody in these films speaks American, is obviously well-nourished, and probably has a haircut that they wouldn't be embarrassed by on the street. Costume design owes less to the V&A Clothing Archive and more to Princess Barbie and/or He Man. Women are financially and emotionally independent and no-one is racist unless they are baddies, and even then everyone else knows that it's not cool, and explains this at least once. Wars, when they occur, do so in fields away from major economic sites and centres of population. This is probably why they also tend not to be particularly dangerous, unless you are a sidekick or have small children.

FP = Frankly Preposterous. At this level, any knowledge of the actual historical events portrayed will be a positive hindrance to understanding what the hell is going on in the film. Errors will be clear to anyone who has been exposed to facts about The Past to the extent of, say, playing Civilization II. There may be a "historical consultant" in the credits, but this person will actually be a nine year-old who has once done a project on castles. The main job of the historical consultant on films like this is to make up new kinds of sword, for example a sword with spikes on it, or a sword with three points or a sword that's, um, really big.

300 = Contains armoured battle rhinos. Apparently.