Perpendicular straight lines in space

Two straight lines $r$ and $s$ are perpendicular if the scalar product of its governing vectors is zero: $r perpendicular to s \Leftrightarrow \vec{u} \cdot \vec{v} = 0$

Example

The straight lines:

$r : (x, y, z) = (0, 0, 0) + k \cdot (3, 2, 4) and s : (x, y, z) = (1, 1, 3) + k \cdot (0, 2, - 1)$

are perpendicular since its governing vectors verify: $(3, 2, 4) \cdot (0, 2, - 1) = 0$