here is a picture of one. i can post a better one when I get my toolbox out of my friends truck. Don't ever leave your tools! They are ok, but I don't feel right without them.

As you can see it is a circle that can be referenced looking straight down upon a full scale lay-out on the floor (the french way). It would plumb out right above the floor lay-out. In this situation it is just acting as a weight on the end of a line. The little wooden piece that holds it is handy as well. Don't have to hold it that way when scribing.