= TeX format
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A \b[TeX format] is a prepared collection of definitions and initialization state used with <TeX>. A <TeX format file>, conventionally ending in `.fmt`, stores internal structures so they can be loaded efficiently instead of reconstructed from the original initialization input.<TeX by Topic by Victor Eijkhout>[§33.1.2: loading and dumping formats]{ref}
<Plain TeX> supplies one such foundation; <LaTeX> supplies another, with document-level conventions and facilities. The same mathematical input `$x^2$` can be embedded in either system, but an authoring command defined by one format need not be available in the other.
Keeping the layers separate helps diagnose a failed document. The <TeX> implementation provides <TeX primitives>; the format makes further <TeX macros> and defaults available; a document invokes those definitions. Loading a different format can change the meaning or availability of commands without changing their spelling in the input file.
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