How to draw lines on 3d object?

[JD Mather]

I usually use DUCS, but you have to understand how it works.

If you don't understand how it works you can be absolutely sure you are on the face by turning off DUCS and use the UCS with Face option (assuming you really do have a solid).

It is well worthwhile to master all of the UCS options if you are going to do 3D.

In fact, it is pretty much a requirement.