TeX is a typesetting system that incorporates a macro processor. A TeX source document specifies or incorporates a number of macro definitions that instruct the TeX engine how to typeset the document. The TeX engine also uses font metrics generated by Metafont, or by any of several other mechanisms that incorporate fonts from other sources into an environment suitable for TeX. TeX has been, and continues, a basis and an inspiration for several other programs, including e-TeX and PDFTeX. The distribution includes the source of Knuth's TeX book; this source is there to read, as an example of writing TeX ; it should not be processed without Knuth's direct permission.