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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.^[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.^[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.

TeX is Donald E. Knuth's programmable system for typography, particularly mathematical typesetting. A document supplies text and instructions; TeX macros and TeX primitives build material whose dimensions determine the eventual page layout. Computer fonts supply glyphs and font metrics, while line breaking selects how a text paragraph is divided. The Knuth-Plass line-breaking algorithm is one of the system's central layout methods.^[ref]

A TeX format supplies a prepared collection of definitions. Plain TeX and LaTeX provide different document interfaces built on TeX technology. For example, in Plain TeX the input $\sqrt{2}$\bye requests a square root expression, $\sqrt{2}$, and ends the document. The computer font, glyph positioning and mathematical typesetting rules determine the printed geometry; the source characters alone are not a picture of the result.

The TeXbook by Donald E. Knuth is the system's user and reference manual. TeX source code by Donald E. Knuth exposes the implementation as a documented computer program, making the paragraph algorithm and its numerical conventions inspectable.^[ref]