This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The method of using the thumb as a gauge on a rule in scribing long boards is not always satisfactory, especially if the board has a rough edge. It is always best to have a regular gauge, but in the absence of one, an attachment for an ordinary carpenter's rule can be quickly made from a piece of tin, although one made of sheet brass is better, in appearance as well as for service. Cut out the metal, as shown by the dimensions, and roll the two sides up, stopping at the dotted lines. The ends A and B are turned out slightly so that they will slide easily along the edge of the board. The gauge will snap on a rule easily and will stay where it is placed. - Contributed by H. J. Blacklidge, San Rafael, Cal.
Ill: Gauge Made of Sheet Metal Which will Easily Snap on a Carpenter's Rule