The LaTeX command for lvert rvert is \lvert x \rvert (absolute value (amsmath preferred)).
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Write \lvert x \rvert inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. This symbol needs the amsmath package, so add \usepackage{amsmath} to your preamble.
Write \lvert x \rvert; for delimiters that grow with their content wrap them in \left and \right, or pick a fixed size with \bigl and \bigr, often the better choice.
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\left( \right)\left[ \right]\left\{ \right\}\langle \rangle\lfloor \rfloor\lceil \rceil\| x \|| x |\lVert x \rVert\left| \right|Delimiters in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \lvert x \rvert in math mode after adding \usepackage{amsmath} to your preamble. For example, $\lvert x \rvert$.
It comes from the amsmath package: add \usepackage{amsmath} to your preamble.