The term pitch traditionally refers to the number of characters from a particular font that will fit in a single inch. GDI, however, uses this term differently. The term fixed-pitch refers to a font whose character-cell size is constant for each character. The term variable-pitch refers to a font whose character cells vary in size, depending on the actual width of the characters.
Variable-pitch fonts use the average character width to specify the average width of character cells in the font. Since there is no variance in character-cell width for fixed-pitch fonts, the average character width specifies the character width of any character in the fixed-pitch font.
Variable-pitch fonts use the maximum character width to specify the maximum width of any character cell in the font. Since there is no variance in character width for fixed-pitch fonts, the maximum character width is equivalent to the average character width in the fixed-pitch font.
When raster fonts are created, they are designed with one particular aspect ratio in mind. The aspect ratio is the ratio of the width and height of a device's pixel. GDI maintains a record of the ideal x-aspect and y-aspect for individual fonts. The ideal x-aspect is the width value from the aspect ratio of the device. The ideal y-aspect is the height value from the aspect ratio of the device. These values are called the digitized aspects for x and y. The GetAspectRatioFilter function retrieves the setting for the current aspect-ratio filter. Windows provides a special filter, the aspect-ratio filter, to select fonts designed for a particular aspect ratio from all of the available fonts. The filter uses the aspect ratio specified by the SetMapperFlags function.
When a particular font is not available on a device, GDI sometimes synthesizes that font. The process of synthesizing may add width or height to an existing font. Whenever GDI synthesizes an italic or bold font from a normal font, extra columns are added to individual character cells in that font. The difference in width (the extra columns) between a string created with the normal font and a string created with the synthesized font is called the overhang.