Generally there are five aspect rations that are commonly used in theatres today.
1.37:1 (often given as 1.33:1) - used for all films made before 1953 (when CinemaScope was introduced) and occasionally still used in Europe, also for some documentaries, 16mm and non-widescreen TV.
1.66:1 - used widely in Europe
1.75:1 - used in the UK, Australia and New Zealand, sometimes in Europe. Close to the digital TV ratio of 16:9 (1.77:1)
1.85:1 - used in America/Canada and other places (particularly if a film is aimed at the US market)
2.35:1 - CinemaScope (also known as Panavision)
Occasionally you might also find...
SuperScope (used on the original
Invasion of the Body Snatchers) and occasional other films like
Days of Heaven
2:1 - used for some obsolete CinemaScope processes
2.2:1 - the ratio of 70mm (which you'll often hear referred to as 65mm, which is the size of the negative - the extra 5mm is the soundtrack)
2.66:1 - used for some obsolete "ultra-wide" processes using 65mm negatives, e.g. "Ben Hur".
Yanman Home Theatre provides an interesting explanation of aspect ratios and "letterboxing" with examples.
