[Ed]

This is the "projection/distribution format" not the "recording format"

Common ratios include:

1.33 - 35mm silent films, standard definition TV, full-screen DVD (4:3)

1.37 - 35mm full-screen, most movies between 1932 and 1953 (Academy) (sometimes incorrectly referred to as 1.33)

1.43 - IMAX (aka 1.44)

1.66 - 35 mm flat, used by several European countries (aka 1.67)

1.78 - full-screen HDTV, anamorphic DVD (16:9)

1.85 - 35mm flat, US and UK standard starting in 1953 (Academy Flat)

2.35 - 35mm anamorphic prior to 1970 (early Panavision, CinemaScope)

2.39 - 35mm anamorphic from 1970 onwards (Panavision, Scope) (aka 2.40, sometimes referred to as 2.35)

[Ed]

For the purposes of "maintaining the original aspect ratio" the following are deemed equivalent as films are transfered to DVDs and BDs.

1.85 and 1.78 - 3.8% loss in image area (sides)
(US and UK theatrical "standard" starting in 1953 to HDTV, Blu-Ray and anamorphic DVD)

1.37 and 1.33 - 2.9% loss in image area (sides)
(Most movies between 1932 and 1953 to SDTV, and non-anamorphic DVD)

2.39 and 2.35 - 1.7% loss in image area (usually top and/or bottom)

People interested in the details will be able to look up the individual ratios. Generally speaking whenever you do not see black bars on your TV the image has been truncated to some extent, except for some silent movies, direct to video, and productions made for TV/HDTV.