= Plain TeX
{c}
\b[Plain TeX] is <Donald E. Knuth>'s foundational collection of <TeX macros>, parameters and conventions, described in <The TeXbook by Donald E. Knuth>. It makes a usable document interface from the lower-level facilities of <TeX>.<Plain TeX in the Comprehensive TeX Archive Network>[Official package description: format-building files and relation to The TeXbook]{ref}
For example, `$x^2+1$\bye` requests an inline mathematical expression and ends the document. The `\bye` macro handles paragraph completion and final output before ending the run.<TeX by Topic by Victor Eijkhout>[§32.1.2: ending a TeX run and the Plain TeX bye macro]{ref} The example uses the format's existing definitions together with <TeX primitives> and <mathematical typesetting> rules.
<Plain TeX> is useful when studying how relatively small definitions control layout. <LaTeX> provides a different <TeX format> with more document-level structure. Choosing a format determines which definitions an input file can expect to find already loaded.
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