The Design of Everyday Rulers

By The Metric Maven

Metric Day Edition

One day my father was helping a relative create a wooden plaque for an extracurricular project. They needed to find the center of a piece of wood, and searched for my father’s centering ruler, but it was nowhere to be found. The relative grabbed a nearby ruler and began to measure the distance, which involved fractions. They began to calculate and confusion began to proliferate. Finally my father said, let’s do it in a way my son would be proud of. My father always carries a high quality tape measure which has inches on one side, but is marked with millimeters on the other (not centimeters).  A quick measurement produced a value of 86 mm which immediately told them that 43 mm was the center.

When he was relating this story to me, my father commented that the centering rulers he owned had inches, picas and other units, but no metric. At first I thought this odd, but I realized that with a mm tape measure, the need for a centering ruler would be essentially eliminated. It also struck me as an example of a tool that one purchases because in the US we use Ye Olde English measures with fractions. Finding the center with millimeters is such a simple calculation, it essentially eliminates the need for a centering ruler (aka center finding ruler). It was yet another example of unnecessary costs involved with using inches and fractions.

When I started trying to use metric at my place of work about a decade ago, I was also required to use inches, so I was stuck with using a dual scale ruler. Below is a scan of the ruler I used for many years:

I had not looked at it in a long time, but I noticed that it has 1/32 inch minimum graduations over the first inch, that is from zero to one inch. After one inch, the minimum graduations shift to 1/16 inch. I looked at another ruler and found it had 1/32 inch graduations on the 0-1 and 11-12 inch sections, but 1/16 in between. I had a hard time understanding how this could be useful. Was this scale to be used only when measuring below 1 inch? If that was the case why have it repeated on the 11-12 inch section of the ruler? When I looked, all three of the different inch rulers I own have the divisions from 0-1 and 11-12 with 32nds and elsewhere 16ths of an inch as the minimum graduation.

I spent some time online looking for an explanation to this 32nd of inch on each end mystery. My six inch rule was consistent in that it only had 32nds from 0-1 inch and had no 11-12 inch section. Why on earth was this done, and it is done very consistently.

One Saturday, when talking with Sven, I pointed out this mystery to him. He took a breath, and then began: “I swear, I once saw a ruler like this.” Sven then drew the ruler on a sheet of paper as an illustration for me. Here is what he drew:

This is a ruler which has only inch marks from zero to five. In other words there are only whole inches from zero to five. From the virtual -1 inch, which is not marked, to zero the graduations are in 32nds. I looked in astonishment and asked: “How on earth would you measure with this?” Sven then explained that you would take whatever object you were going to measure, and line it up with the closest inch so that the other end would dangle between -1 and 0 inches. One would then figure out the fractional overlap and add that to the integer inch value. I was dumbfounded, clearly that would work. It was complicated in comparison to a millimeter only metric ruler because of the choice of halves, quarters, eighths,  sixteenths, and 32nds, but it would work.

This method could be used with a common ruler, but a person would have to remember to subtract 1 from the inch graduation chosen, and then add on the fractional part. This seemed far too complicated for use by the average citizen, but I see no other possible way to readily use the 32nds scale at both ends. Millimeter only rulers will immediately provide an integer value which is precise to within 1/25th of an inch, and with some judgement, 1/50th of an inch. No need to sort out fractions, just read an integer from a “number line.”

I have mentioned Donald A. Norman’s book The Design of Everyday Things in previous essays. He has an idea called affordance. This is the idea that a product should have designed into it clues as to how it should be used. In the case of the dual end 32nds it does not seem to immediately suggest how one might make use of the scale. Sven’s ruler at least gives you a bit of a clue about how to use it from the fact there are no fractional divisions from 0 to 5. My dual scale ruler also has a clue as to which side should be preferred. One can only read the Dual-Rule logo and the text which documents who manufactured it, when using the inch scale. Despite the inch preference, this was the ruler I really could not give up, because when you turned it over, it was the only ruler I had which had a millimeter only scale on it.

Others were all cm/mm rulers. This was my old reliable.

It would not be until Mike Joy sent me 150 mm, 300 mm, and 600 mm  single scale metric millimeter only rulers from Australia that I would finally be able to retire this dual scale ruler. I don’t use it anymore in my engineering work, but for some reason I keep it on my desk, perhaps because it was the only millimeter-only scale I had for many years.

As I have pointed out previously, the Invisible Metric Embargo in the US makes it very hard to obtain mm only rulers and tape measures. For many, many, years my father has carried around a small dual-scale tape measure. This tape measure is the same one used to find the center of the piece of wood related earlier. My father’s tape measure is unusual because the metric side is in millimeters. Unfortunately, his tape measure finally wore out. The scale of the first 100 mm or so was worn off to the point it had become illegible. My father called all the technical suppliers he knows—and he knows a lot of them. He had no luck finding a replacement. The tape measure had been discontinued. My father was becoming desperate. He never did find a new replacement. He had, however, purchased an extra tape measure over twenty years ago with his original order, and serendipitously found it. Here is what the “new” tape looks like:

Dual Scale Tape with millimeters on top (click to enlarge)

This tape measure is truly a metric anachronism in the US. The tape measure lists meters first on its front (3m-10′) and the millimeter scale is on the top, with inches below. This tape measure suggests metric, with millimeters, and de-emphasizes Ye Olde English. Centimeter/millimeter tapes are common, millimeter-only tapes are sighted slightly less often than Bigfoot and Elvis.

It was brought to my attention that there is a place in the US where one can purchase an American-made millimeter only rule. I ordered one, and after it arrived, realized that ruler designers in the US are essentially Mormons making coffee. Here is a photo of the US ruler and an Australian counterpart:

Australian 300 mm rule (top) and American made 300 mm rule (bottom)

I’m sure my readers realize that the Australian rule is very legible, and the US ruler, despite the high contrast, has such uniformly small numbers, that it is very hard to read. The US design appears to have almost zero thought put into it. This is why I end up obtaining rules from Australia.

One day I received an email from Peter Goodyear. In the email he stated he had purchased a dual-scale metric ruler that weekend. It seemed like a joke, and I treated it as such. Peter then wrote back to tell me it was no joke, and sent me a scan of it. I was stunned. Proof that he had not Photoshopped his way to this ruler arrived in the mail a week or two later from Australia. Here is an image of the ruler:

I was absolutely floored when I looked at it. What I realized was that even something seemingly as simple as the design of a ruler actually has a number of options. The millimeter side of this dual-scale ruler has millimeter graduation lengths which become longer as one approaches half-way and then retreat in the same manner as they approach the next whole number millimeter designation. I’m still thinking about if I think this is a good idea or not—I’m leaning toward not. Sven is not impressed, and would leave things as they are implemented on millimeter-only metric rulers, with half marks and the other designations of equal length.

The remarkable side of the ruler Peter sent me is the centimeter side. I’m perhaps deriving a bit of schadenfreude at the dismal treatment the centimeter is given. It’s just seen as a sort of close-enough for no actual work dimension. It only has half centimeter graduations, and no finer. The centimeter side reminded me of part of an aphorism about workmanship I heard in Montana that I will alter slightly: “He’s the kind of guy that would measure to a millimeter, mark it with chalk and cut it with an axe.”  Well, in this case there’s no need, there is a dual scale ruler for that, and he can use the centimeter side.

Pat Naughtin used to say “Don’t dual with dual.”  I could not agree more. Use millimeter only metric rulers, without a dual-scale, metric or Ye Olde English, and life will be good.

As I was revising this essay, I received a 300 mm long, millimeter only, metric ruler from Peter Goodyear. It looked ordinary enough, but when I flipped it over, the backside had a millimeter only centering ruler!  This came as a considerable surprise. After looking for a mm only centering ruler in the U.S. for a long time, one would show up serendipitously from Australia. Here is what it looks like in case you are curious:

Australian metric centering rule

I noted a setback for metrication this year when I came across this set of calipers in a large electronics retailer:

Calipers with a readout for fractional inches

I’ve never seen an electronic calipers with direct fractional readout before. You will note it is the first option listed on the package. It strikes me as equally useful as offering calculators with an option to have an output in Roman Numerals. It does have a mm scale on the slide, and a very legible readout for them.

This is my third metric day essay, and usually I cannot find a millimeter of change in the US, but this year is different. As I stated above, the only metric millimeter only rule I could find in the US, and manufactured in the US is poorly designed and I cannot recommend it. But this year the Japanese company SHINWA is offering mm only metric rules on a US based website, and I have purchased a 150 mm and a 300 mm ruler. They are actually a bit different from all the Australian rules I have. The 50, 100, and 150 mm labels are in red. The 300 mm rule has them at 100, 200 and 300 mm. The front of the 150 mm rule is given below:

Front of Shinwa H-101A 150 mm Ruler — click to enlarge

These rulers are not just imports it appears. The back of the ruler has tables which are clearly aimed at US users:

Back side of Shinwa H-101A 150 mm ruler — click to enlarge

Yes, it has a table of whole inches to millimeters, fractions to decimals, and non-metric tap information. The rulers have no dual rule markings, no centimeters, inches, or barleycorns, just millimeters. The rulers are as high quality of manufacture as any of my Australian rules.

Here’s wishing you the best on Metric Day. I say this with a minute amount of hope that next year I can report that I purchased a satisfactory millimeter only metric tape measure from a US based website. Until then the only option I know about is the slightly less than satisfactory True 32 (blue case) tape measure.

Postscript 2018-11-02

I encountered an image of a ruler that is marked in a manner different than all the options I’ve seen here or in my essay “The American Metric Ruler.”

Here centimeters are shown as a coarse length, and are labeled alphabetically as CM. They are passively discouraged as they do not have divisions. The numerical (Hindu-Arabic) labeling is given only as millimeters, but not labeled alphabetically. The user is assumed to understand the numerical values are millimeters without explanation. This is another example of poor design choice. All the information a user needs, assuming they are completely new to the device, should appear.

Related essays:

Stickin’ it to Yardsticks

The American “Metric Ruler”

America’s Fractional Mind

If you liked this essay and wish to support the work of The Metric Maven, please visit his Patreon Page and contribute. Also purchase his books about the metric system:

The first book is titled: Our Crumbling Invisible Infrastructure. It is a succinct set of essays  that explain why the absence of the metric system in the US is detrimental to our personal heath and our economy. These essays are separately available for free on my website,  but the book has them all in one place in print. The book may be purchased from Amazon here.

The second book is titled The Dimensions of the Cosmos. It takes the metric prefixes from yotta to Yocto and uses each metric prefix to describe a metric world. The book has a considerable number of color images to compliment the prose. It has been receiving good reviews. I think would be a great reference for US science teachers. It has a considerable number of scientific factoids and anecdotes that I believe would be of considerable educational use. It is available from Amazon here.

The third book is called Death By A Thousand Cuts, A Secret History of the Metric System in The United States. This monograph explains how we have been unable to legally deal with weights and measures in the United States from George Washington, to our current day. This book is also available on Amazon here.

20 thoughts on “The Design of Everyday Rulers

  1. It’s interesting that you didn’t mention inch scales other than the usual binary 1/2, 1/4 etc reducing to 1/32.

    When I was at elementary school I had a 12-inch ruler with scales for tenths and twelfths as well as the usual eighths and sixteenths.

    • Yes, I commonly work in a mix of metric and the obsolete units and found a tape measure that makes the best of it. A Stanley 33-272. (available via Amazon). When forced to work in Inches, using tenths does reduce errors.

      This has an even stranger bit – the bottom scale is in ‘metric inch’ where the first 6″ are labeled in 1/10th-inch and divided in 2/100ths. The rest of the tape is only divided to 1/10″

      Common, non machine shop rules and tape measures, are created via some printing method. And errors build over the length – which could also explain this short length of higher resolution. (It is only accurate beyond a 1/10″ for 6″) Or it could be to increase the yield due to problems in printer technology printing these fine lines? (Someone with printing experience might fill in the limits of accuracy and resolution with common printing methods).

      I have a different rule that is divided in 1/100th. (.254mm) I was just able to read laser lines on this rule to this accuracy.

      In search of a corresponding metric ruler – I have never seen 0.1mm of course, but not even 0.2 mm divisions – which I’m sure could be hard to manufacture, but could be useful.

      And yes – the new availability of metric rules is something to rejoice – a positive indicator that the USA is definitely and freely going metric. I suspect that in the next decades, there will be a tipping point when inches will suddenly been seen as nothing other than a historical curiosity. Perhaps then, someone will make a tape measure where the only a few indications for 16″ centers remain for maintaining old houses.

      And then – perhaps someone could design a replacement connector for garden hoses – metric, with a bayonet twist lock sealed with a reliable O-ring?

  2. “These rulers are not just imports it appears. The back of the ruler has tables which are clearly aimed at US users:”

    My take on it is that there is manufacturing outside the U.S. that caters to the American desire of products that are inch based. Case in point: I design PC boards in mm, send the gerbers to China (the American way, you know) for fabrication. When they are delivered, the invoice talks about the dimensions of the PC boards in inches. I’m pretty sure they do that for their Yankee customers.

    • I’ve seen the same thing – the idea that the US is less metric than most of the so-called metric countries if just not true. As they see more and more US designs specified in metric, they too will move to metric. One could even say that the USA is starting to drive the metrication of the world.

      My local hardware store, on rearranging the nuts, bolts and screw section – has even more of the space devoted to metric than before.

      • That is because with the plethora of imported products, people will come in and ask for a replacement screw and when it turns out to be metric and the shop doesn’t carry it, they lose a sale. Also, how many of the hardware store bolts are sold for automotive usage even if the property class is incorrect?

    • Do the boards, when measured equal a millimetre value or an inch value? If you contacted the company and told them to describe them only in metric, would they do it? Don’t guess their reason, ask them why they do it that way?

  3. It must be my eyes, but I find the American-made 300 mm ruler with white marks and numbers on a black background extraordinarily readable. Who made it? Are there other drawbacks not evident in the photo?

    • John S and Others:

      The QSI Corporation (now Beijer Electronics, apparently) makes (made?) 250-mm plastic rulers with mm-by-mm black markings on a white front (numbered by tens, of course) along with a list of the 20 prefixes underneath (guess the Maven would want only 16, but perhaps he’d let it go here), all of which is easy to read.

      [I had obtained such through the USMA. Thus, maybe Paul T could let us know if such is still available through USMA.]

    • Thanks again to Peter Goodyear for helping me identify the source. My PEC Tool black chrome 300 mm metric ruler arrived. I find it very well made and exceptionally easy to read. The numbers are slightly smaller than on the Australian rule in the photo above but larger than on many of my other rules. I think it is my new favorite ruler.

      We all have our preferences. If the Maven prefers larger numbers that is fine, but I think it VERY unfair to say “almost no thought went into it.” I find it a well made and very legible ruler. PEC makes MANY measuring instruments. Here is a link to their black chrome line; however if you like black on shiny, they make that too. See links to other pages in left corner.
      Note that all their rules are offered in various graduations, metric, binary inches, decimal inches, and various combinations (4 scales on a ruler). Be sure to order metric/metric if that is what you want.

  4. A couple of comments on the Maven’s National Metric Day essay:

    – When I was in primary school in the 1960s, I noticed the rulers we used had inch markings and centimeter/millimeter markings on the other side. However, it appears over the last thirty-or-so years that many rulers for such pupils have only an inch scale sans anything on the other side, which of course is disconcerting. (I see this in ads and in stores selling school supplies.)
    – Re: “The millimeter side of this dual-scale ruler has millimeter graduation lengths which [that!] become longer as one approaches half-way and then retreat in the same manner as they approach the next whole number millimeter designation.”
    My somewhat biased comment: As someone with a background in statistical science, this is something I really liked to see! (After all, Maven and Sven, just what harm does such do?)

  5. A few years back I was in Mexico on a project. Every person at the company I encountered had only a dual tape measure. There was metric on the bottom and inches on the top.

    Like this:$_12.JPG

    Most people falsely think the usable side needs to be on the top. The Mexicans had no idea what inches were and never used that portion of the tape. But when I discussed the issue with them, they preferred the millimetres to be on the bottom if not on the whole tape.

    Americans would hold the tape in their right hand, they would clip the end to whatever they are measuring or have someone hold it and pull to the right and use the top part of the tape to mark with their left hand. For those who write with their right hand, this is clumsy.

    The Mexicans did the opposite. They preferred to hold the tape with their left hand and pull to the left. The numbers, even though upside down were not a problem, but it allowed them to mark the dimension with the right hand, which was what most used for writing.

    Dual measuring cups are the same way. The cups are made so that the preferred side faces you when holding with the right hand. But this can be awkward if you need to pour something into the cup while holding it. It forces you to leave the cup on the counter and bend over to see the markings so you can pour with your right hand.

    A person using the cup who wants to pour something into the cup while holding it needs to hold it in their left hand and thus in most cases this puts the metric side in front of you, and makes it easy to pour into with the right hand.

  6. I’ve just seen this blog and it’s interesting to see from a European point of view. My mind boggles a little – I would NOT have like to have maths and physics done in anything other than SI units. g is 9.8m/s/s right? Right?

    When I was a kid in school we used 300mm rulers that had inches and cm on them – the inches were subdivided into 10ths, not 16ths. That’s still the case from what I see but tech drawing is mm only (eg: I’ve never been taught in anything but SI units.

    Here in Ireland we had a long term hangover from the older UK system which made some of our measurement systems very odd indeed. People still measure their weight in stone (14 pounds) for example which Americans find very confusing. We also had British Imperial, Irish Instantor and metric plumbing systems; often all three simultaneously in the same house!

    The problem here was one of choice – the UK went construction metric in the 1970s while Ireland went with “whatever you want”. Thus a complete lack of coherence and a generational divide – older kept with imperial and younger went metric. This has faded now but plumbers all carry imperial olives and adapters to compensate.

    I’ve seen “15mm” and “22mm” fittings stickered over with 1/2 inch and 3/4 inch labels. 15mm pipe will not fit into a 1/2″ fitting but 1/2″ pipe will fit into a 15mm fitting – this drives people like me completely nuts. In Africa, this goes on all the time with “1/2 inch pipe” varying depending on whether it was imported from the US or from Europe and relabelled. And vice versa with 15mm pipe.

    As for liquid medicines (an earlier post of yours about tsp and tbl) I’ve one here for kids (Calpol paracetamol – what they call acetaminophen in the US). It comes with a little plastic spoon in it with “5ml” stamped on it. Solves that problem straightaway. The infant formulation comes with a little plastic dosing syringe (and an app!).

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