A Staff Report from the Straight Dope Science Advisory Board

On tape measures, why is there a special symbol every 19-3/16 inches?

September 24, 1999

Dear Straight Dope:

Some of my friends recently noticed that on most tape measures there is a diamond every 19 3/16 inches. What is the purpose of this mark?

Well, Stephen, as a rule, hand tools are pretty self-explanatory. But this one stumped me, so I went to http://www.stanleyworks.com to see if they addressed this issue. Sure enough, you're not alone in wanting to know the answer to this question, as its inclusion in the FAQ section indicates. Here's Stanley's answer:

The black diamond marks every 19.2" on our tape rule blades are for spacing Engineered Lumber joists and studs. Several wood product manufacturers offer Engineered Lumber as a substitute for conventional lumber. Span tables for these lumber products provide ratings for spacing of 12in., 16in., 19-3/16in., and 24in. If you multiply these dimensions by 8, 6, 5 and 4, respectively, you'll notice that you come up with 96in., the length of the panels that will be used for sub-flooring or sheathing. Engineered Lumber is usually specified by the architects and engineers who draw the plans. They take advantage of its strength by using fewer joists or studs where codes allow.

So there you have it, Stephen. Those little diamonds are there so you can remember quickly what one-fifth of 96 is. Not that you need to remember that very often, but who knows, maybe this is all the impetus you need to go out and get that contractor's license.

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