Principles of a Framing Square

Square Corners

• The most basic principle of the framing square is that of the square, or right angled corner. The two arms of the square, the larger blade and smaller tongue, meet at the heel or outside corner at a 90-degree angle. This is a square corner since the angle in both directions is equal. To use the square to align a right angle, you place the heel on the inside of the corner and align the two pieces with the outside edges of the blade and tongue.

Plumbing with a Square

• A vertical right angle, or plumb, is another important carpentry principle that you can measure with a framing square. Positioning the square with the tongue on the edge of the horizontal plane, whether it is a floor or some other surface, and the heel positioned against the base of the vertical piece to get the plumb. Align the vertical piece with the outside of blade with the edge of the tongue on the horizontal. Once aligned, it is said to be plumb or "true". This indicates that the upright is perfectly straight vertically.

Calculating Angles

• You can calculate any angle, such as the angle of a roof pitch or stair stringer, with a square. Determine the horizontal distance from the bottom of the angle to a point directly below the highest point. For example, the top of a wall directly below the roof's peak. You can also find the distance in inches and divide by 12 to convert to feet. To find the height from the point below the highest point to the point in inches, you have to divide the height by the number of feet of horizontal run. You need to mark the horizontal run on the tongue at 12 inches, mark the divided height in inches on the blade, and tie a string between the two points to find the angle.

The Rule of Three, Four and Five

• You can extend the square beyond the ends of the physical square by using the principle of the rule of three, four and five. Set the square with the heel at the corner of any structure, such as a fence line. Measure out in line with the tongue of the square a distance divisible by three, it can be divided by 3 inches, or 3 feet, and mark the point. Measure out in line with the blade a distance divisible by four, use inches or feet, and the same multiplier you chose for the tongue. Mark the point.

  Measure between the two points. If your lines extend straight, the resulting distance will be equally divisible by five. Adjust your lines as needed to arrive at the correct distance. For example, if you measure to 45 inches along the tongue, measure to 60 inches along the blade, then the diagonal distance between the two lines, if they are at 90 degrees, will be 75 inches.