L - Commands  
LINE_PT (command)
   Overview   Operations  Commands 

It creates a straight line perpendicular to a curve and tangent to an other one

 Command line: LINE_PT

Specify the first line vertexon object, near the perpendicular point: Specify a point

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

 

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