L - Commands  
LINE_PP (command)
   Overview   Operations  Commands 

It creates a straight line perpendicular to two curves

 Command line: LINE_PP

Specify the first line vertex on object, near the perpendicular point: Specify a point

Specify the second line vertex on object, using the Perpendicular Snap: Specify a point

 

The action is completed using a Deferred Perpendicular Snap for the first requested point, see also Using the Object Snaps