\buildrel - LaTeX accent command

\buildrel a+b \over {c+d}

Renderer support

Package	MathJax	KaTeX	XeTeX
`base`	MathJax: Full	KaTeX: None	XeTeX: Full

Syntax#

\buildrelu{\over}a\overmath contentm{b}math content

Argspec u{\over} m — two mandatory arguments. Reading argspecs

\buildrel \rm def\over =

\buildrel{a\over b}\over=

TeXForm parses this entry as prefix syntax; the package determines its argument specification.

\buildrel #1 \over #2 \mathrel{\mathop{#2}\limits^{#1}}

MathJax directly constructs an munderover node wrapped in a TeXAtom of class REL.

Rewrite plain TeX buildrel syntax to stackrel.

\buildrel #1 \over #2 \stackrel{#1}{#2}

\stackrel{#1}{#2}approximate
Achieves a similar visual stack but produces different MathML structure and uses standard brace-delimited arguments.
\mathrel{\overset{#1}{#2}}exact
This matches MathJax's output including the surrounding relation-class TeXAtom.
\overset{#1}{#2}approximate
Without an added \mathrel, \overset does not itself force relation spacing/classification.

`base`
Input	Case	MathJax	KaTeX	XeTeX
`a \buildrel \rm def \over = b`	Positive · ctx:basic	Pass	Fail	Pass