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I have an old LCD monitor by Dell. Its model number is 2407WFPHC and its resolution is 1920 x 1200.

I attach it to my modern Surface Laptop 5 whose resolution is 2256 x 1504. In the System Settings -> Display window, when I select the "Extend these displays" option, I see fullscreen windows on both laptop and monitor displays but when I select the "Duplicate these displays" option, I see a small black left-out area on left and right sides, on the attached monitor. I assume this is due to the fact that the maximum supported resolution for the external monitor is lower than the laptop's so the monitor can fill only 1920 x 1200 pixels on the screen, but then how can it fill the entire screen when in extended display mode? What is this phenomenon called? See the picture, the red arrows point to the black areas.

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    Pillarboxing, or vertical letterboxing or reverse letterboxing.
    – Andrew Morton
    Commented Jan 28 at 9:42
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    For exactly this reason when I set up a multiple monitor configuration, I try to use monitors with the same native resolution, even if they are different physical sizes (e.g., 1920x1080 with 24" and 27" screens). That makes it so no windows need to change when moved between screens. Of course, laptops often complicate this with more unusual native resolutions.
    – manassehkatz-Moving 2 Codidact
    Commented Jan 29 at 1:45
  • @AndrewMorton that should be an answer! It’s more accurate than the current answer
    – Josh
    Commented Jan 29 at 4:54
  • @Josh It's OK, Jörg added the info to his answer.
    – Andrew Morton
    Commented Jan 29 at 9:17

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What is this phenomena called?

It is not quite 100% clear what you are asking about, but I guess the best answer would be "basic geometry". In television and movies, it is called pillarboxing, but according to my observations, this term is not well-known in IT. A more well-known term is letterboxing which refers to the dual: adding blank space at the top and bottom in order to display wide content on a narrow screen.

The Surface screen is 2256 pixels wide by 1504 pixels high, that is a width-to-height aspect ratio of 2256:1504 = 3:2 = 1.5.

The external monitor is 1920 pixels wide by 1080 pixels high, that is a width-to-height aspect ratio of 1920:1080 = 16:9 = 1.77777777….

No matter how you scale two rectangles with different aspect ratios, it is impossible to fully cover one with the other.

Or, simply put: the external screen is wider than the Surface screen, so it is impossible to display the same content on both without:

  • blank space on the side(s) of the external screen,
  • blank space on the bottom, top, or both of the Surface screen,
  • cutting off the top, bottom, or both on the content on the external screen, or
  • cutting off the content on the side(s) of the Surface screen.

All this assumes only scaling. If you are willing to stretch or squeeze the content, then it can fill both screens, but it would look "wrong" on at least one of them. In particular, a circle would turn into an oval on at least one of the two screens.

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    To apply this to the original question, in "extend" mode the screens can have different screen ratios, but in "duplicate" mode they must have the same ratio. I suspect if OP makes the external monitor the primary display, it will fill the monitor screen, and show black bars on the laptop screen.
    – kevin
    Commented Jan 29 at 0:51
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    I think more people would be familiar with the term "letterboxing". It might make sense here, even though it refers to blank space at the top/bottom, rather than left/right.
    – Roger Lipscombe
    Commented Jan 29 at 9:10
  • Additionally, the original opposite to letterboxing is "picket-fencing" where the black bars are vertically. Since the advent of phone video, it has become more known as "portrait mode"
    – Criggie
    Commented Jan 29 at 23:06
  • I've never heard either of these terms. Another term that some folks might be more familiar with is "underscan". The opposite is "overscan" which is a widely accepted term used in some Linux distros and the MacOS docs.
    – tjohnson
    Commented Jan 30 at 21:24
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    @tjohnson: underscan and overscan haven't been a thing in decades. They are specific to CRTs and the OP clearly states their external monitor is an LCD, not a CRT. CRTs are really only used in retro-gaming nowadays.
    – Jörg W Mittag
    Commented Jan 30 at 21:29

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