A ratio is a proportional comparison of usually two but sometimes more entities or characteristics and is often written out as numbers separated by a colon (the colon serves as a substitute for the word "to") or, if the ratio is only two numbers, as the quotient of the two numbers (the quotient of two numbers is the result of the division of the two numbers) .
The aspect ratio of a rectangle refers to the ratio of its width-to-height . Since images, videos, and displays typically take the form of rectangles, this definition applies to images, videos, and displays as well.
Some issues to consider when writing out ratios are how precise to be when defining the ratio, whether to use a number comparison or a quotient, and how much to reduce the ratio if a number comparison is being used.
The first issue is how precise to be when defining a ratio. One key example of this is analog film aspect ratios, which tend to be given in approximations due in part to inconsistent dimension information and minor dimension variations over the years     . Another example of this is so-called 21:9 (2.(3)) displays, which actually have an aspect ratio of 64:27 (2.(370)) or 43:18 (2.3(8)) depending on the display resolution being used . However, with the exceptions of analog film and 21:9 displays, approximations will be avoided unless otherwise noted.
The next issue is whether to use a number comparison or a quotient. Analog film aspect ratios typically use quotients due to their approximate nature and also to make it easier to compare different aspect ratios. For other ratios, both a number comparison and a quotient will be used initially but on subsequent uses only the number comparison will be used.
The last issue is how much to reduce the ratio if a number comparison is being used. Normally, one would simplify the numbers in a ratio as much as possible. However, when ratios are compared with other ratios, one or more of the ratios may be presented in a non-simplified form. For example, although the 21:9 ratio used to describe 21:9 displays simplifies to 7:3, 21:9 is often used instead to make for an easier comparison with 16:9 (1.(7)) displays. That being said, if a ratio has a commonly used non-simplified number comparison, this non-simplified number comparison will be mentioned only on initial use. Subsequent mentions will use only the simplified form. For example, this means that from now on the 21:9 aspect ratio will be referred to as a 7:3 aspect ratio.
There is disagreement concerning how to refer to PAR, RAR, and VAR as well as what terms certain acronyms should be used for. This is shown in the table below, with only the first word of the term used since the last two words of all of these terms are aspect ratio    :
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Resolutions with non-square pixels will be indicated by linking them back to their earlier article of explanation.
1. Ratio. Wikipedia.
2. Aspect ratio. Wikipedia.
3. 35 mm film. Wikipedia.
4. Martin Hart. Of Apertures and Aspect Ratios. (C) 1997-2000 The American WideScreen Museum.
5. Martin Hart. Relative Frame Dimensions. (C) 1997-2012 The American WideScreen Museum.
6. Martin Hart. CinemaScope: Facts On The Aspect Ratio. (C) 2004 The American WideScreen Museum.
7. Martin Hart. ASPECT RATIO and USEABLE IMAGE AREA of 35mm and 70mm Motion Picture Prints. (C) 2005-2012 The American WideScreen Museum.
8. 21:9 aspect ratio. Wikipedia.
9. Aspect ratio (image). Wikipedia.
10. Pixel aspect ratio. Wikipedia.
11. Thread started by shun on July 21, 2010 1:54 AM. please explain SAR/DAR/PAR. (C) 1999-2015 videohelp.com.
12. Aspect ratios. Last modified on Sept. 19, 2014 5:30 AM. AviSynth Wiki.