One of the straight lines sweeping out a ruled surface. The rulings on a ruled surface are asymptotic curves.
Ruling
See also
Asymptotic Curve, Director Curve, Ruled Surface, Ruled Surface DirectrixExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Ruling." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Ruling.html