Measure and mark for better accuracy
Establish a definitive rule
Every country has a bureau of standards, which maintains a set of incredibly precise measuring instruments against which other measuring devices are calibrated. Carry this concept into your workshop. Choose a precise, finely etched 12" steel rule for the bulk of your measuring tasks. A quality combination square provides not only a rule; the head, with 90° and 45° angles built in, increases the tool's versatility [see Proving a square has the right angle, below]. For measurements longer than 12", use only one 12' tape measure. Anything longer is just extra bulk to carry. And check the tape against the rule to make sure they agree, right.
Now that you've established which rule rules the roost, make sure everything else in your shop agrees with it; for example, the rip-fence indicators on your bandsaw and tablesaw, center right, and any other rulers. If other rulers don't measure up, relegate them to the house.
A metric rule can come in handy, too, especially if calculations with imperial dimensions give you a headache. For example, determining one-half, one-fourth, one-fifth, or three times 7 7/8" brings out the pencil and paper compared with working with its metric equivalent, 20cm.
Proving a square has the right angle
Calling a tool a square doesn't make it square. To ensure that yours lives up to its billing, do this simple test with a piece of straight-edged scrap.
With the head of the tool to one side, draw a line the length of the blade. Flip the square and draw a second line next to the first. If the second line parallels the first, bottom two photos, the square is true. If the lines slant away from each other, the square needs adjustment or replacement.
After determining that your square is square, protect it from drops and bumps that could compromise its accuracy.
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