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  Video images consist of a large number of closely spaced pixels. The
  display resolution is defined by the number of pixel rows, or scan lines,
  from top to bottom and the number of pixels from left to right in each
  scan line. The horizontal and vertical resolution is limited by the
  capabilities of the video monitor as well as the display circuitry inside
  the computer. The video modes available on the different subsystems were
  carefully designed so that the horizontal and vertical resolution in each
  mode is within the limits imposed by the hardware.

  The MDA's single text mode has 720 x 350 pixel resolution; that is, the
  screen has 350 scan lines, each of which contains 720 pixels. Because 25
  rows of 80 characters of text are displayed in this mode, each character
  is 9 pixels wide (720 . 80) and 14 pixels high (350 . 25). The CGA's text
  modes are a bit lower resolution, because the CGA's pixel resolution is
  only 640 x 200. Thus the 25 rows of 80-character text on a CGA consist of
  characters that are only 8 pixels wide (640 . 80) and 8 pixels high (200 .
  25). That's why text looks sharper on an MDA screen than on a CGA.

  The trend in the newer IBM video subsystems is to provide better vertical
  resolution. For example, the EGA's 80 x 25 text mode has 640 x 350 pixel
  resolution, so text characters are 8 x 14 pixels. On the MCGA, the default
  80 x 25 text mode has 640 x 400 resolution (8 x 16 characters), and on the
  VGA the same text mode has 720 x 400 resolution, so characters are each 9
  pixels wide and 16 pixels high. From a program's point of view, the 80 x
  25 text mode is the same on the CGA, the MCGA, and the VGA--it's display
  mode 3 in all cases--but a user sees much higher resolution when using a
  VGA or MCGA than when using one of the older subsystems.

  You see the same trend towards better resolution when you examine the
  graphics modes available with the newer video subsystems. The VGA's 640 x
  480, 16-color mode has more than twice as many pixels on the screen as the
  original CGA's 640 x 200 graphics mode. It's ironic that this CGA mode was
  known as a "high-resolution" mode when the CGA was new.