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According to Wikipedia:

The name “4K resolution” refers to a horizontal resolution of approximately 4,000 pixels.

However, I believed that 4K is called like this simply because it has four times the resolution of a 1080p video, like this:

enter image description here

and, well, 1080 is practically 1024, which is a round number 1K.

Personally, the Wikipedia's explanation doesn't make much sense to me: the resolution of a piece of video is usually specified by two things: height and ratio; it is very unusual to see any mention of the width.

So what is the real origin of “4K” term?

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  • Analogue video is defined by the vertical resolution, e.g. 625/50 being 625 lines at 50Hz, AKA PAL. This is because there is no horizontal resolution per se, the horizontal being a continuous line. Digital video was originally defined like this, e.g. 720p and 1080p, but once we hit 2000 pixels across it started to be all about the horizontal resolution. This is probably because it sounds bigger when the TV salesperson says it.
    – stib
    Aug 16, 2016 at 15:50

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According to Wikipedia ,Digital Cinema Initiatives originated the term 2K in 2002 for a digital film format of 2048 x 1080. 2048 is commonly referred to as 2K in the digital world. It logically follows that when DCI went on to specify a resolution 2x larger in each direction, they called the 4096 x 2160 format 4K. 4096 is commonly referred to as 4K in the digital world.

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  • Especially as many "4K" systems are not actually 4096 pixels wide.
    – paulzag
    Aug 10, 2016 at 5:08
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    The OP inquired as to the origin of the term 4K. He did not inquire as to who might, or why they might, exaggerate their claims that 3840 horizontal pixels of the UHD format is "4K". We all know the answer to that one. Marketing folks have always rounded off numbers to favor greater claims. Those who do care about accuracy in specifications know the difference between UHD and 4K.
    – Michael Tiemann
    Aug 10, 2016 at 12:49
  • The common TV format referred to as "4K" has an active video area of 3840 H x 2160 V pixels. There are additional pixels during the horizontal and vertical blanking periods, which are of course unseen. However, in the Digital Cinema world the format known generically as "4K" is 4096 H x 2160 V active video pixels. In TV-Land frame rates are usually 60 FPS, sometimes 30 or 15. But in Digital-Cinema-Land the frame rate is usually 24 or 48 FPS, which relate to conventional mechanical cinema which uses a 24 frames per second picture source (e.g. 35 mm film), but flashes/strobes each frame twice.
    – FiddyOhm
    Aug 16, 2016 at 14:05

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