Instructions for the Lufkin Red End 656 Brick Masons Rule: To take a measurement when morter thickness is known: As an example, assume mortar thickness is 1/2" and the average thickness of brick is 2-1/4". Added they would give a 2-3/4" course. Simply place your thumbnail on the edge of the rule at 2-3/4", turn over to the other side, and your thumbnail will be on the scale 6 mark. This is the correct scale to use under these conditions. If only one size is known, for instance only the brick size is known, use the closest mark. For instance, assume that a space 37" is to be bricked. Using the brick mason's scale side of the rule to measure, the 37" figure falls between scale 4 and scale 8. Checking both of these scales, by the previously described thumbnail method, at the starting end of the rule, it is found that scale 4 is equal to 2-5/8" and scale 8 is equal to 2-7/8". If the brick averages 2-1/2", scale 4 would allow 1/8" joint and scale 8 a 3/8" joint. The more satisfactory of these two size joint thicknesses can be selected for the job.