This is your plum line. I picked this up at our local hardware store. It is used to make sure that your 4x4's are square. Now how this works is that you put in your plum line. This is where your fence is going to go. The other function of a plum line is to tell you where your posts are going to go. Thirdly when you use your plum line make sure that this point on your post and this point on your post are touching equally the plum line. We line up all your 4x4's in a straight line to make sure that they are all plum and when you put on your framing everything will be square for your picket fence. Another function of your plum line is to establish the height of your picket fence. From one end to the other I put a level on it to make sure that this is level and when you are done with the framing of the fence we take our pickets and we attach the pickets to the frame to this height. That way you will know that your finished product of your picket fence is level all the way down.