A directional accent placing a right-pointing arrow above text to indicate morphisms or directed segments in category theory.
Examples
Vector notation for directed line segment
\underrightarrow{AB} = \underrightarrow{BA}
Directed path in geometry
\text{Path } \underrightarrow{PQ} \text{ has length } d(P,Q)
Vector with multiple components
\underrightarrow{ABC} = \underrightarrow{AB} + \underrightarrow{BC}