Line1: A(10,15,20) to B(50,40,15). Line2: C(15,10,40) to D(45,35,25). Find the shortest distance between these non-intersecting lines.

Draw projections of both lines in FV and TV
Check if lines intersect - if not, they are skew lines
Shortest distance is perpendicular to both lines simultaneously
Draw auxiliary view showing one line as a point
In this view, measure perpendicular distance to second line
This is the shortest distance
Vector method: d = |(B-A)·((C-A)×(D-C))| / |(D-C)×(B-A)|