The LaTeX command for emdash (—) is --- (em dash).
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Write --- in normal text, outside math mode. It is part of core LaTeX, so no extra package is required.
Type --- in running text, outside math mode. With XeLaTeX or LuaLaTeX you can also type — directly.
Open this example in the live preview: it renders as you type, with no compiler and no signup.
\dag\ddag\S\P\textbullet\dots--\textasciitilde\textasciicircum\textbackslash\textasteriskcentered\slash``''Text symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use --- in normal text; it needs no extra package. For example, ---.
None: --- is built into LaTeX, so no extra package is required.