In desktop publishing and digital prepress, the automatic replacement of an indicated but unavailable font in a document with an available font that most closely resembles it. A program that practices automatic font substitution analyzes the characteristics of the desired font—weight, syle, size, etc.—and attempts the best match it can. This may or may not generate good results, and a poor substitution can dramatically alter a page layout.
Additionally, fonts files may be installed and/or managed differently on a Macintosh computer compared with a PC running a Windows OS. Font substitution occurs when a font file has gone "missing," or is not installed, or is not compatible with a different OS. When layout files are moved from one computer or platform to another, it is essential that all font files used in those documents are copied and installed as well. Font substitution may be caused by failure to follow this procedure. Thorough understanding of font licensing is needed to avoid illegal use of fonts in such a situation.