Sometimes it's easier to use the Fillet-command to join polylines. Start Fillet, click the first polyline, hold down the shift-key while clicking the second polyline. Done! The shift-key makes a fillet with radius 0, a corner. This also works with joining a polyline to a line. And this is also great if the endpoints are not on top of each other, but visibly are apart, the fillet will extend and trim the lines at their intersection. The Chamfer-command will also do the sam thing (with the shift-key). Try it out to see if it is suitable for your problems. Good luck