Can't trim my lines

If you have drawn your linework in full 3D, meaning you have elevations assigned to the ends of the lines, then the 2 lines probably do not intersect so the trim command will not work. The lines must both be drawn at the same elevations. Please see our technical note about drawing 2D vs. 3D.

MSCAD2005 - you can now trim lines with different elevations.‚  You may need to set the Trim command to use the UCS or VIEW option under projection.‚  start the Trim command, pick the first line, then type in P for Projection and set it to View or UCS, as appropriate.

This may also need to be set to allow Contour Labeling to work correctly.