The mitre joint is one of them ... http://en.wikipedia.org/wiki/Woodworking_joints
These kind of joints. I don't know the english word for plastic / steel joints. Actually I don't even know the word in my own language ;-)

With coordinate system I simply mean a little x/y/z symbol so I know where is the Y axis. Sometimes my viewport is so crowded and when I zoom in, with a construction plane active I loose my orientation.