Aspect Ratio is how the video industry describes the shape of your screen. Older, CRT-based screens looked mostly square, so they had a 4:3 aspect ratio. What this means is that for every 4 pixels in width the screen had, it had 3 pixels in height. So those screens are 33% wider than they are tall.
Modern, LCD-based screens are rectangular and have a 16:9 aspect ratio. This means that every 16 pixels in width are accompanied by 9 pixels in height. Or that the screens are nearly twice as wide as they are tall. See the figure below for an illustration.
This is important because if the aspect ratio for your media is not the same as the aspect ratio for your screen, you will have black bars either above and below or to the sides of your media (letterboxing and pillarboxing, respectively). If you ever bought a Widescreen-formatted DVD and tried to play it on a CRT TV, you have probably experienced this first-hand.
If you have your displays mounted vertically but your media is playing in the middle with large black bars above and below the media, your media was made for a 16:9 aspect ratio and your screen is trying to shoehorn that into your 9:16 screen (remember, it's mounted vertically!). To fix this, you either have to switch your screen to be mounted vertically or reformat your media to take advantage of a vertical screen.
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