WYSIWYG means that the screen output matches the printer output. With perfect WYSIWYG, the user would be able to place a page of printed output over the same screen output and see every character and graphic element in exactly the same place. If the screen and printer have different resolutions, however, this degree of matching is impossible. Usually, WYSIWYG simply means that line breaks, paragraph breaks, and page breaks are the same on both devices and that justified paragraphs are presented properly. WYSIWYG does not mean the same document on two different printers will be formatted in exactly the same way. Because most applications make the best use of the available printer, WYSIWYG often applies only to the correspondence between the screen and printer for a given printer.
TrueType offers a higher level of WYSIWYG than was available with earlier versions of Windows, because it works on every device. Most Windows applications lay out the screen based on the target printer. The fonts they enumerate for the user are the fonts that can be printed. Because TrueType fonts work on the target printer, they are enumerated by the printer driver to the application and are typically displayed to the user as printer fonts. When the application and GDI match screen fonts to the printer fonts, the TrueType fonts are used on the screen as well.
If no screen font matches the widths of characters in the chosen printer font, WYSIWYG is difficult to achieve. When this happens, applications sometimes make the average width of the characters match, with as little variation in specific characters as possible. More exact matching is achieved with a technique known as metric coercion. There are two basic methods of coercing character metrics: width coercion and shape coercion. Width coercion simply adjusts the spacing between words and characters, and shape coercion applies a transformation to each character to force it into a bounding box. Because shape coercion can lead to unacceptably deformed characters, width coercion is typographically preferred.
Although Windows does not include a function to deform individual characters, the lfWidth member of the LOGFONT structure allows an application to scale the width of a TrueType font independently of its height. (Most applications do not scale TrueType fonts in this manner, however, because the results are usually unattractive.)