I is for Interior Folding Rule

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I is for Interior Folding Rule

Click to enlarge - it's worth a close look!
These little babies are pretty obscure. To be honest, I don't have much call to use them, but they are so interesting, I'm glad I have them. As you can see from the photo, folding rules with an extension section have been around for a while, but continue to be made. The small one dates from the 1950s and belonged to my grandfather. The larger, newer one is much more recent - 1995? 2000?

Unlike Allen wrenches, drill presses, hammers, and many of the other tools I've been discussing, the interior folding rule has one function, which it does supremely well. It's a real specialty item, used for measuring the exact interior dimensions of closed frames, such as windows, door jambs, boxes, dressers, etc. Why is it so specialized? Allow me to (briefly) explain.

Folding rules are the precursors of modern tape measures. Any carpenter always had a folding rule in his kit. In the old days, a "tape" measure meant a long piece of cloth tape, 25, 50 or 100 feet, coiled around a hub or in a small round drum. Any distance shorter than about 10 feet was measured with a folding rule. Any distance longer than 10 feet used multiple increments of the folding rule or (since that led to errors) a single use of the tape(1).

Even after the innovation of the modern tape measure - a thin tape made of retractable, flexible spring steel - a problem remained. How do you measure the inside of something? This sounds like a dumb question, but it's more pertinent than you might guess.

If you measure from edge to edge on the outside, you're only interpolating about what the measurement is on the inside, based on the assumption that everything is straight, square and plumb. Good luck with that, Skippy.

On the other hand, if you put your folding rule (or tape measure) down inside and measure directly, you can put the far end flush against one side, but you have to hold the rule at an angle against the other. That means you must estimate the true measure based on an eyeball of where the rule is in relation to the far end(2). Again, you can be off by as much as 1/8", the difference between square and crap.

Enter the interior folding rule. It goes inside the space to be measured and is unfolded into as many units as the space will fit. What's left over is measured with the extensible brass slide. The rule is placed flush against one side; the extension is slid out until it's flush against the other side. Length of rule units + length of extension = interior space width.

The REALLY great thing about this tool is that it allows you to take incremental measurements of the interior space. In a bright, shiny world where every doorjamb is a perfect rectangle  and every window slides easily in a perfectly square frame, this is unimportant. In the real world, however, where doors and windows stick and bind, it's pretty useful to know the real shape of the materials you have to work with. As you run the interior folding rule down the space, you can slide the brass extension in and out to account for the changes of the space, in 1/16" increments.

I haven't had to rebuild window frames in a while, but I just recently did some cabinet work where this tool came in handy. As I said above, it's a specialty item but is an absolute champion at what it does.

1. REALLY long distances needed surveyor's cord, but I won't get into that.
2. Some tape measures note the width of the tape itself so that you can extend the tape, take a reading and add the tape width. In my experience, those width measurements are not very accurate, and get less so as a tape measure gets dinged around.
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11 comments:

  1. My dad has one of these and they're really useful. I've even used one to measure the inside of a shoe so I know where to cut any removable insoles.

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    1. Yay! I'm really glad to have someone else with experience using one of these, and I'm doubly glad it's you, Icy. After you've used one, the instant impulse is to stare at it and think, "Damn, that's really clever!"

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    2. Ours belonged to my great-grandfather who was a pattern maker.

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  2. Gotta respect a tool that is still useful and being produced after decades or (probably) over a century. I wonder how long it will be before someone invents a laser-based version of one of these, if it hasn't been done already. A premium version might even record a series of measurements (say, if you slide it down the frame) to note deviations.

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    1. There are laser rules that do this, but in my experience, they are pretty fussy to use.

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  3. Huh. I'm not sure I've ever worked with one of these before, but I can see where you'd get use out of it. Measuring the inside of things has gotten painful for me at times.

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    1. It's perfect for that one job. Gotta love the specialization.

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  4. My dad had one like the yellow one in your photo. When I was young I used it as an unfolding gun - pistol or rifle depending upon the need. It makes great spy gear. :)

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    1. I did the same thing with my grandfather's!

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  5. This comment has been removed by a blog administrator.

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  6. I love those folding rules fiercely! Apart from any practical use they may have--and they have many--it is so satisfying to fold and unfold them, and just to see them sitting there, partially unfolded, full of measuring promise. Thanks for the memories!

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