The LaTeX command for triangleleft (◁) is \triangleleft.
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Write \triangleleft inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
As a binary operator, \triangleleft gets a medium space on each side; at the start of a formula LaTeX reads it as a sign and drops the space. With unicode-math under XeLaTeX or LuaLaTeX, typing ◁ in a formula gives the same symbol.
$a \triangleleft b$Open this example in the live preview: it renders as you type, with no compiler and no signup.
Binary operators in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \triangleleft in math mode; it needs no extra package. For example, $a \triangleleft b$.
None: \triangleleft is built into LaTeX, so no extra package is required.