A bidirectional arrow accent placed over text or variables to indicate vector equilibrium or reversible processes in physics.
Examples
Line segment with two endpoints
\overleftrightarrow{AB} \text{ represents the line through points A and B}
Vector equilibrium in physics
\overleftrightarrow{PQ} = \overleftrightarrow{QR} \text{ indicates equal and opposite vectors}
Geometric line with distance
\text{Length of } \overleftrightarrow{CD} = 5 \text{ units}