The numbers in a screen resolution determine the aspect ratio. This term describes the screen's/image's shape.  For instance, the aspect ratio on old box televisions was 1:1, which means that the screen's horizontal and vertical lengths were the same. Meanwhile, because most films are shot in a rectangular aspect ratio, square TVs had to cut a portion of the image or blackout a part of the screen to fit everything. The aspect ratios of 1920x1080 and 1366x768 are both 16:9. They are rectangular, with the horizontal length nearly twice as long as the vertical length. The differences in aspect ratio between these two aren't easily noticeable. However, when you use multiple screens, some differences start to appear. Using both 1980x1080 and 1366x768 displays will result in minor shape distortions in the images. For many applications, these differences are insignificant; however, if you require high-precision images, the aspect ratio can be an issue.