Tipster Eng Cheong shows us a trick when trimming overlapping lines in AutoCAD.

"When you want to trim a line that is overlapping say, a rectangle, but this line is located behind the object, then you are likely to select the item in front by mistake. Use the command Bring to Front (DrawOrder) to bring it to the front before trimming it.

"There is also a sequence you can use to skip that step and go straight to the trimming:

1. Select the rectangle as the cutting element
2. Use a crossing polygon to select both the overlapping line and the rectangle (cutting element)
3. Now you're finished: The line has been trimmed, but the cutting element has not."

Notes from Cadalyst Tip Patrol: The Trim command is a classic tool in AutoCAD. It cuts selected lines at a cutting edge line. Start the command, select the cutting edge line (or lines), then pick the lines you want to trim. You can select more than one cutting edge line at a time. In this tip, the user is faced with overlapping lines, making it difficult to pick the proper cutting edge line. As the tipster suggests, you can get around this by picking them all. You can trim an object to more than one line during your command; just keep picking the line to be trimmed until it is trimmed enough.