Alternatively, a line can be given by two points, which yields a slightly
different definition:
Given two curves ${\bf X}(t)$ and ${\bf Y}(t)$,
a ruled surface is the image of the function
$$
f(s,t) = (1s) {\bf X}(t) + s{\bf Y}(t).
$$
This corresponds to Olivier's process of threading a string through holes
in two brass "curves".
