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“21:9” (“twenty-one by nine” or “twenty-one to nine”) is a consumer electronics (CE) marketing term to describe the ultrawide aspect ratio of 64:27 (2.370:1 or 21.3:9), designed to show films recorded in CinemaScope and equivalent modern anamorphic formats. The main benefit of this screen aspect ratio is a constant display height when displaying other content with a lesser aspect ratio.
The 64:27 aspect ratio of “21:9” is an extension of the existing video aspect ratios 4:3 (SDTV) and 16:9 (HDTV), as it is the third power of 4:3, where 16:9 of traditional HDTV is 4:3 squared. This allows electronic scalers and optical anamorphic lenses to use an easily implementable 4:3 (1.3:1) scaling factor.

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“21:9”
The term “21:9” was chosen as a marketing term, first used by Philips in January 2009. Due to its common denominator, 21:9 is more relatable to 16:9, the aspect ratio of regular HDTVs, rather than the more accurate 64:27. If it actually were 21:9 (2.3:1), the fraction could also be expressed in the reduced form as 7:3, relating to the 4:3 of standard-definition TVs.
Consumer TVs with this aspect ratio were manufactured mainly from 2010 to 2017. Due to it causing pillarboxing with standard 16:9 content, and the resulting low consumer acceptance, this screen format has rarely been used since then.
It is still prevalent in projection systems, using anamorphic lenses, and supported by a number of consumer electronics devices, including Blu-ray players and video scalers.
It is also used in computer monitors, where the term “21:9” can represent aspect ratios of 43:18 (2.38:1 or 21.5:9) and 12:5 (2.4:1 or 21.6:9) in addition to 64:27. The wider screen provides advantages in multitasking as well as a more immersive gaming experience, and even wider screens with aspect ratios such as 32:9 (allowing for two 16:9 views side-by-side) are available. 21:9 phones also exist.