When I mentioned using a caliper when I was having trouble with proportions in an earlier post a couple of art friends asked me about it. I used a caliper years ago in architectural & drafting classes. I pull them out once in a while when I'm drawing, but they have sharp metal tips so I was always worried about putting a hole in a canvas. Artist friend Deena Hunkler-Sanks took a class that used an Accurasee Proportional Divider made from plastic. It helps artists by helping with scale. You align the subject matter you are drawing or painting in the small end of the divider and use the larger end on your drawing surface to get the correct proportion. Artist friend Maria Bennett Hock order one and what a lovely surprise when she gave me one too. It is a very nifty tool, but initially I wasn't using it correctly. I've learned you have to remember to hold your arm out straight EVERY time. Deena was kind enough to send out the video (below) on of how to use it and I thought I would share it with you.
Youtube: Bjorn demonstrates how to draw using the Accurasee Proportional Divider (FYI - A number of the online art stores carry Accurasee Dividers)