question: Why is HD referred to as 1080P, which is it's vertical resolution (1920x1080), while UHD is referred to as 4K, which is its horizontal resolution (3840 x 2160)?
comment: It's just different naming conventions. "4k" sounds more marketable than "2160p". "2k" was actually the short-lived marketing name for 2160x1440 resolution. It's even more confusing because originally "4k" was specifically used to refer to 4096x2160 resolution, but over time TV manufacturers simply started using the 4k resolution to sell TVs which were actually "UHD," which is the official name for 3840x2160. So now 4k refers to either one. I personally prefer to use the vertical resolution to name what resolution I'm talking about. "480p, 720p, 1080p, 1440p, 2160p, etc." Is a lot easier to understand than "DVD, HD, FHD, QHD/2k, UHD/4k, etc."
passages: Although the UHDTV standard does not define any official names for the formats it defines, ITU typically uses the terms "4K", "4K UHD", or "4K UHDTV" to refer to the 38402160 system in public announcements and press releases ("8K" for the 76804320 system). In some of ITU's other standards documents, the terms "UHDTV1" and "UHDTV2" are used as shorthand.
  
  Section::::4K standards and terminology.:CEA Ultra HD.
   This resolution, sometimes referred to as 4K UHD or 4K2K, has a 16:9 aspect ratio and 8,294,400 pixels. It is double the size of Full HD (19201080) in both dimensions for a total of four times as many pixels, and triple the size of HD (1280720) in both dimensions for a total of nine times as many pixels. 38402160 was chosen as the resolution of the UHDTV1 format defined in SMPTE ST 2036-1, as well as the 4K UHDTV system defined in ITU-R BT.2020 and the UHD-1 broadcast standard from DVB. It is also the minimum resolution requirement for CEA's definition of an "Ultra HD" display. Prior to the publication of these standards, it was sometimes casually referred to as "QFHD" (Quad Full HD).
   This resolution, commonly referred to as 5K or 5K3K, has a 16:9 aspect ratio and 14,745,600 pixels. Although it is not established by any of the UHDTV standards, some manufacturers such as Dell have referred to it as UHD+. It is exactly double the pixel count of QHD (25601440) in both dimensions for a total of four times as many pixels, and is 33% larger than 4K UHD (38402160) in both dimensions for a total of 1. times as many pixels. The line count of 2880 is also the least common multiple of 480 and 576, the scanline count of NTSC and PAL, respectively. Such a resolution can vertically scale SD content to fit by natural numbers (6 for NTSC and 5 for PAL). Horizontal scaling of SD is always fractional (non-anamorphic: 5.33...5.47, anamorphic: 7.11...7.29).
   Pixel shifting, as described here, was pioneered in the consumer space by JVC, and later in the commercial space by Epson. That said, it isn't the same thing as "true" 4K. More recently, some DLP projectors claim 4K UHD (which the JVCs and Epsons do not claim).
   Section::::Resolutions.
  
  Section::::Resolutions.:38402160.
  
  The resolution of is the dominant 4K resolution used in the consumer media and display industries. This is the resolution of the UHDTV1 format defined in SMPTE ST 2036-1, as well as the 4K UHDTV format defined by ITU-R in Rec. 2020, and is also the minimum resolution for CEA's definition of "Ultra HD" displays and projectors. The resolution of was also chosen by the DVB Project for their 4K broadcasting standard, UHD-1.
   BULLET::::- 38401080 (3.1 or 329); this resolution was first used in the Samsung C49HG70, a curved gaming monitor released in 2017. This resolution is equivalent to dual 1080p displays (19201080) side-by-side, but with no border interrupting the image. It is also exactly one half of a 4K UHD (38402160) display. Samsung refers to this resolution as "DFHD" (Dual Full HD).
  
  Section::::Recording.
   BULLET::::- 38402400 (1.601 or 1610); this resolution was used in the IBM T220 and T221 monitors, released in 2001 and 2002 respectively. This resolution is also referred to as "WQUXGA", and is four times the resolution of WUXGA (19201200)
   Some 4K resolutions, like , are often casually referred to as "2160p". This name follows from the previous naming convention used by HDTV and SDTV formats, which refer to a format by the number of pixels/lines along the vertical axis (such as "1080p" for progressive scan, or "480i" for the 480-line interlaced SDTV formats) rather than the horizontal pixel count (≈4000 or "4K" for ).
   In 2007, the Society of Motion Picture and Television Engineers published SMPTE ST 2036-1, which defines parameters for two UHDTV systems called UHDTV1 and UHDTV2. The standard defines the following characteristics for these systems:
  
  BULLET::::- A resolution of 38402160 (UHDTV1) or 76804320 (UHDTV2)
  
  BULLET::::- Square () pixels, for an overall image aspect ratio of
  
  BULLET::::- A framerate of 23.976, 24, 25, 29.97, 30, 50, 59.94, 60, 100, 119.88, or 120Hz with progressive scan
  
  BULLET::::- RGB, 4:4:4, 4:2:2, or 4:2:0 pixel encoding
  
  BULLET::::- 10bpc (30bit/px) or 12bpc (36bit/px) color depth
   This resolution has an aspect ratio of 169, with 8,294,400 total pixels. It is exactly double the horizontal and vertical resolution of 1080p () for a total of 4 times as many pixels, and triple the horizontal and vertical resolution of 720p () for a total of 9 times as many pixels. It is sometimes referred to as "2160p", based on the naming patterns established by the previous 720p and 1080p HDTV standards.
   Some articles claim that the terms "2K" and "4K" were coined by DCI and refer exclusively to the 2K and 4K formats defined in the DCI standard. However, usage of these terms in the cinema industry predates the publication of the DCI standard, and they are generally understood as casual terms for any resolution approximately 2000 or 4000 pixels in width, rather than names for specific resolutions.
  
  Section::::4K standards and terminology.:SMPTE UHDTV Standard.
   TVL is defined as the maximum number of alternating light and dark vertical lines that can be resolved per picture height. A resolution of 400 TVL means that 200 distinct dark vertical lines and 200 distinct white vertical lines can be counted over a horizontal span equal to the height of the picture. For example, on monitor with 400 TVL, 200 vertical dark lines can be counted over width on monitor (Note that the of monitor height is used rather than the of whole monitor width).
   In October 2012, the Consumer Electronics Association (CEA) announced their definition of the term Ultra High-Definition (or Ultra HD) for use with marketing consumer display devices. CEA defines an "Ultra HD" product as a TV, monitor, or projector with the following characteristics:
  
  BULLET::::- A resolution of 38402160 or larger
  
  BULLET::::- An aspect ratio of 1.1 () or wider
  
  BULLET::::- Support for color depth of 8bpc (24bit/px) or higher
  
  BULLET::::- At least one HDMI input capable of supporting 38402160 at 24, 30, and 60Hz progressive scan (though not necessarily with RGB / 4:4:4 color), and HDCP2.2
   The term "2160p" could be applied to any format with a height of 2160 pixels, but it is most commonly used in reference to the 4K UHDTV resolution of due to its association with the well-known 720p and 1080p HDTV formats. Although is both a 4K resolution and a 2160p resolution, these terms cannot always be used interchangeably since not all 4K resolutions are 2160 pixels tall, and not all 2160p resolutions are ≈4000 pixels wide. However, some companies have begun using the term "4K" to describe devices with support for a 2160p resolution, even if it is not close to 4000 pixels wide. For example, many "4K" dash cams only support a resolution of (43); although this is a 2160p resolution, it is not a 4K resolution. Samsung also released a () TV, but marketed it as a "4K" TV despite its 5K-class resolution.
   Ultra-high-definition television is also known as Ultra HD, UHD, UHDTV and 4K. In Japan, 8K UHDTV will be known as Super Hi-Vision since Hi-Vision was the term used in Japan for HDTV. In the consumer electronics market companies had previously only used the term 4K at the 2012 CES but that had changed to "Ultra HD" during the 2013 CES. The "Ultra HD" term is an umbrella term that was selected by the Consumer Electronics Association after extensive consumer research, as the term has also been established with the introduction of "Ultra HD Blu-ray".
  
  Section::::Technical details.
  
  Section::::Technical details.:Resolution.
   BULLET::::- Capable of processing images according to the color space defined in ITU-R BT.709
  
  BULLET::::- Capable of upscaling HD content (i.e. 720p / 1080p)
  
  The CEA definition does allow manufacturers to use other terms—such as "4K"—alongside the Ultra HD logo. Since the resolution in CEA's definition is only a minimum requirement, displays with higher resolutions such as 40962160 or 51202880 also qualify as "Ultra HD" displays, provided they meet the other requirements.
  
  Section::::4K standards and terminology.:2160p resolution.
   In analogue contexts, this resolution is often called "525 lines". It is mandated by CCIR Systems M and J, which are usually paired with NTSC color - which led to the "NTSC" name being often inaccurately used to refer to this video mode. Other color encodings have also been used with System M, notably PAL-M in Brazil.
  
  Section::::Technical details.
   On June 24, 2014, the CEA updated the guidelines for Ultra High-Definition and released guidelines for Connected Ultra High-Definition, adding support for internet video delivered with HEVC. The CEA is developing a UHD logo for voluntary use by companies that make products that meet CEA guidelines. The CEA also clarified that "Ultra High-Definition", "Ultra HD", or "UHD" can be used with other modifiers and gave an example with "Ultra High-Definition TV 4K".
   On October 18, 2012, the Consumer Electronics Association (CEA) announced that it had been unanimously agreed on by a vote of the CEA's Board of Industry Leaders that the term "Ultra High-Definition", or "Ultra HD", would be used for displays that have a resolution of at least 8 megapixels with a vertical resolution of at least 2,160 pixels and a horizontal resolution of at least 3,840 pixels. The Ultra HD label also requires the display to have an aspect ratio of 16:9 or wider and to have at least one digital input that can carry and present a native video signal of 3840×2160 without having to rely on a video scaler. Sony announced they would market their 4K products as "4K Ultra High-Definition (4K UHD)".
   Ultra-high-definition television (also known as Super Hi-Vision, Ultra HD television, UltraHD, UHDTV, or UHD) includes 4K UHD (2160p) and 8K UHD (4320p), which are two digital video formats proposed by NHK Science & Technology Research Laboratories and defined and approved by the International Telecommunication Union (ITU). The Consumer Electronics Association announced on 17 October 2012, that "Ultra High Definition", or "Ultra HD", would be used for displays that have an aspect ratio of at least 16:9 and at least one digital input capable of carrying and presenting native video at a minimum resolution of 3840×2160 pixels.
  
  Section::::Sets.:Market share.
   The "UHD Alliance", an industry consortium of content creators, distributors, and hardware manufacturers, announced Monday on the 11th of January 2016 during CES 2016 press conference its "Ultra HD Premium" specification, which defines resolution, bit depth, color gamut, high-dynamic-range imaging (HDRI) and rendering (HDRR) required for Ultra HD (UHDTV) content and displays to carry their Ultra HD Premium logo.
   Section::::Adoption.:DVB.
  
  In 2014, the Digital Video Broadcasting Project released a new set of standards intended to guide the implementation of high resolution content in broadcast television. Dubbed DVB-UHDTV, it establishes two standards, known as UHD-1 (for 4K content) and UHD-2 (for 8K content). These standards use resolutions of 38402160 and 76804320 respectively, with framerates of up to 60Hz, color depth up to 10bpc (30bit/px), and HEVC encoding for transmission. DVB is currently focusing on the implementation of the UHD-1 standard.
   In 1993, Texas Instruments built a 2048x1152 DMD prototype. No rationale is exposed in the papers for choosing this specific resolution over the Japanese 1035 active lines system, or alternatively doubling the 480 lines of the standard US TV to 960, but that way it could cover all resolutions expected to be present on the market, and that included the European one, which happened to be the highest. Some legacy of this development may be seen in "2K" and "4K" digital movie projectors using TI DLP chips, which run a slightly wider than usual 2048x1080 or 4096x2160 resolution, giving 1.896:1 aspect ratio without anamorphic stretching (vs the 1.778:1 of regular 16:9, with 1920 or 3840 horizontal pixels), give a little (6.7%) more horizontal resolution with anamorphic lenses when showing 2.21:1 (or wider) movies specifically prepared for them, and further enhancement (~13.78%) through reduced letterboxing if used "without" such lenses.
   BULLET::::- 38401600 (2.401 or ); a number of computer monitors with this resolution have been produced to date, the first of which was the LG 38UC99-W released in 2016. This resolution is equivalent to WQXGA (25601600) extended in width by 50%, or 38402160 reduced in height by ≈26%. LG refers to this resolution as "WQHD+" (Wide Quad HD+), while Acer uses the term "UW-QHD+" (Ultra-wide Quad HD+) and some media outlets have begun using the term "UW4K" (Ultra-wide 4K).
   Section::::See also.
  
  TV transmission systems
  
  BULLET::::- Analog high-definition television systems
  
  BULLET::::- PAL, what MAC technology tried to replace
  
  BULLET::::- SECAM, what MAC technology tried to replace
  
  BULLET::::- A-MAC
  
  BULLET::::- B-MAC
  
  BULLET::::- C-MAC
  
  BULLET::::- D-MAC
  
  BULLET::::- E-MAC
  
  BULLET::::- S-MAC
  
  BULLET::::- D2-MAC
  
  BULLET::::- HD-MAC, an early high-definition television standard allowing for 2048×1152 resolution.
  
  BULLET::::- DVB-S, MAC technology was replaced by this standard
  
  BULLET::::- DVB-T, MAC technology was replaced by this standard
  
  Related standards:
  
  BULLET::::- NICAM-like audio coding is used in the HD-MAC system.
  
  BULLET::::- Chroma subsampling in TV indicated as 4:2:2, 4:1:1 etc...
  
  Section::::External links.
  