Quote Originally Posted by David A. Goldfarb
Clearly, over there they shoot verticals with 5x4 and 10x8, while we shoot horizontals, but everyone shoots horizontal with 5x7.
To add to the confusion:

On mainland Europe we (/they) use metric sizes. Instead of 4x5" it's 9x12cm, 5x7" is replaced by 13x18cm, 8x10" by 18x24cm and so on.

Further confusion: A sheet of 13x18cm film measures almost exactly 5in x 7in, and so is a little bit larger than a sheet of 5x7" film. "A little bit" in this case means "just enough bigger that theyre not interchangable". But not enough that the difference is easily seen. You only notice when the film jams when you're trying to load it in the holder (13x18 in 5x7 holder), or drops out when you insert the dark slide after exposing the film (5x7 in 13x18 holder). I know this, because I've tried it both ways (Norway gets both sets of sizes)...

A vertical conundrum: As you see, we always state the short dimension first. Yet nearly all the cameras I've seen that are old enough to have fixed backs (like my 1934 Voigtländer Bergheil 9x12 plate camera) are vertical.

So we shoot verticals with 9x12, 13x18 etc., and the British shoot horizontals with their 5x4, 5x7 and 10x8's...