For scaleable fonts, Windows may be able to change the "aspect ratio" (the normal height-to-width relationship of the characters). As a result, some scaleable fonts can be used with an unusually large or small number of characters per inch by stretching or squeezing the image of each character horizontally.
Because this stretching and squeezing can make a scaleable font less attractive, the GDI Mode lets you specify a pitch of "zero characters per inch" using the command [ESC](s0H. Windows will then select a font based on height alone and calculate an appropriate width to ensure that the character images do not appear distorted.