Dual Unit On One Unit


By The Metric Maven

There are proverbial questions that seem abundantly obvious as they appear to contain the answer within the question itself. These questions are sometimes offered as jokes. For instance, Groucho Marx would ask contestants  on You Bet Your life questions like: “Who’s buried in Grant’s Tomb?” Despite the obvious nature of the question, the answer is “no one is buried in Grant’s Tomb.” Both Grant and his wife are in sarcophagi above ground and not buried at all. While the question seemed obvious,  there was an unexpected subtlety to the answer.

Recently I was looking at site statistics for themetricmaven.com and came across the list of the top twenty search strings which ushered people to the website. I have the first 15 included below:


Notice that millimeter ruler, and mm ruler are number one and two. Number nine is “how to read mm tape measure.” To a lot of metric proponents this may seem as oddly obvious a question as such as: “how long did the Hundred Years War last?” But in the US, the design of rulers make this far less than a conceptual slam dunk. For instance here is a ruler that was photographed for a project in Nuts and Volts, an electronics magazine:

The metric side is in millimeters it appears, but the graduations are chosen to split the digits with a virtual decimal point to create integer centimeters, virtual decimal centimeters, centimeters with millimeters or integer millimeters. It is hard to make out, but there is a handy set of printed instructions which appear to read:

To read length in centimeters omit the final zero after the index line. To read length in millimeters include the final zero.

This may seem like a rather redundant and perhaps even intelligence insulting set of instructions. But in the US, unfamiliarity with the metric system is so ubiquitous, that poorly marked centimeter/millimeter rulers, which have only mm labeled on them, cause confusion between centimeters and millimeters. I’ve written about this in my essay The American Metric Ruler.

I wondered who had manufactured this ruler and so I wrote to Nuts and Volts and the contributing editor. I did not receive a reply. The ruler seemed strangely familiar and then it struck me, it’s some variation of a Starrett ruler. I have a 1000 mm Starrett ruler. On its front side it has inches and millimeters in the same way as the ruler shown above, but no instructions. When I looked at the back side of the ruler, it appears to be a millimeter only metric ruler from 0 to 1000 mm. It also has the instructions for use on it:


This is a Starrett Aluminum Meter Stick No. MS-2. It has 33 conversions written below the millimeter only scale, so it is clearly aimed at US users.

I find it quite an oddity that we have metric rulers which are dual scale like this one from a previous post:


This ruler is one of the very few that identifies that both units are on the same ruler. Rather than just choose millimeters and make this ruler a single unit ruler, we have dual unit rulers in the US. Starrett tries to cut the baby in half by placing an index line so that both units are defined on the same ruler, a sort of dual-unit on one unit ruler.

The visceral clutching onto the centimeter, which is far too large for any ordinary precision work, is exasperating. As I’ve pointed out, some rulers put the centimeter into perspective by having a centimeter side and a millimeter side. Here is what the centimeter side of a ruler like this looks like:


In the US the centimeter is treated as just another version of an inch, but it is not. The inch is divided using fractions which are not of identical numerical scale (i.e. they cannot be directly added like integers) but are theoretically the same unit. The centimeter is a unit that is too large for use by itself, and so in the US one immediately uses decimals; but this is equivalent to the same integer number in millimeters, with the addition of an extra unnecessary symbol—a decimal point. One can decimalize centimeters in an attempt to preserve something like Þe Olde English inch, using centimeters and decimals, which are analogous to inches and fractions, or one can choose a unit which is simple for everyday use—millimeters. No instructions needed. The irony for me is that I was constantly told in grade school to choose the “right unit” when I was schooled in medieval units, such as the inch, foot, yard and mile; but metric is so esoteric in the US, that it seems nonsensical to my fellow citizens to use millimeters alone and to mark rulers with them. With this much confusion and dogma inculcated into everyone, it should not be surprising that Americans would need instructions on how to read a millimeter ruler, as they so seldom ever see one.

Related essays:

The Design of Everyday Rulers

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.

7 thoughts on “Dual Unit On One Unit

  1. I think that young American students need to have a decimal fire lit under them. The ability to conquer the SI through visualization of the units is what is needed, I.e. a teacher who points to the graduations on a metric ruler and explains how to “see” the centimeters among the millimeters without any labels. Sadly, I don’t think we teach this skill. The budding user of the SI should be given the chance to think decimally, and, hopefully, yes, choose millimeters for short lengths.

    • I agree with this. My “inch” rulers never identify that the big numbers are inches, they identify the size of smallest graduation. My favorite inch ruler has 4 scales marked 32nds, 64ths, 1/10, and 1/50. Obviously the six big numbers (it is a 6″ ruler) are inches.

      Similarly, my favorite metric ruler has the same two scales on each side, mm and 0.5 mm; the size of the smallest graduations. Obviously 10 of the little mm marks ( or 20 of the 0.5 mm marks) add up to 10 mm = 1 cm.

      Perhaps teachers need to teach better but those who find this an insurmountable obstacle probably can’t read an inch-based ruler either, because the markings are “similar.”

      I do have some rulers and tapes where a short segment has finer graduations (in inches, 1/32″ for a distance, followed by 1/16″ is common) but inch and metric rulers with a constant graduation size are identified by smallest graduation. Metric Maven: The wooden ruler in your photo should really be identified as mm and 0.5 cm, NOT cm.

  2. ¡Hehehehe….! This sounds as using the “HISPANIC” exclamation mark –> ¡ <— We have not been pushing enough to make politicians understand we are practically behind the rest of the world with respect to measurement. A TV chief meteorologist from ABC7 (WZVN) in Fort Myers, Florida wrote to me in an inquiry I made to him and I quote: "At the present time, I do not envision television weather converting to metric for the simple reason that the official unit of measure in the United States is the Imperial Unit, not metric."… "While we, as meteorologists and scientists, use the metric system in our work, our viewers, the general population, do not."
    Many times, we are accustomed to blame the "American people", but the real shameful administrators are those individuals working for government agencies like the NIST, FHWA, NASA, NOAAA, etc, etc, etc, etc, etc, etc, etc,…(¿More et ceteras? — Hihihihi…) I have also been insulted in some occasions by respectful people from respectful metric system organizations and I have been called a "liability" just because I have been encouraging those officials to be more active in terms of disseminating even more educational information to the American people. ¿Is Donald Trump right in his anti-immigration speeches? ¿Is he really a RACIST? ¿Why are still many people following him even though and supposedly he is not going to be nominated as candidate for the presidential chair in 2016?
    What I´m going to say now is not going to like to the USMA: There is no one "Hispanic" individual as "officer" in the USMA organization. So, discrimination and racism is still very strong in the U.S, thus "discrimination" against the International System of Units (SI) or the "much-maligned-in-America Base-Ten System", the Metric System, is considered by most American people as an "Alien", an invader to our land coming from strange countries. ¿Who we are going to blame for failing to be a "metric nation" in the second decade of the 12st Century? The answer cost the price of U.S. debt!

    • Oh my God…! You have read my comment! I apologize for my impertinent commentary but I´m getting older and I my tolerance with many administrators in Federal Agencies is almost zero at this time. I have been battling something that it is practically impossible because of the brutality and corruption in our society. I have said in the past we need to do even more and I strongly believe I have done too much for the USMA cause at this time in the Fort Myers area, Florida. I have been offended and treated disrespectfully by some colleagues of mine in many occasions without their concerns in the educational process. On the other hand, I am glad to have some achievements in my “metric crusade” since I decided to fight ignorance on our educational system about this “much-maligned-in-America Base-Ten System”.

  3. MM thanks for another interesting read..
    Manufactures of rulers and measuring tapes, have little tricks to make metric rulers and tapes hard to read. This discourages some people, particularly older people whose eyesight may not be as good as when they were younger.
    Notice how on the black ruler, that the metric millimeter markings are equal length. The 1, 2, 3, 4, 6, 7, 8, and 9, markings are the same length.
    Compare that to the markings on the inch scale, where the 1 inch markings are longer than the 1/4 inch markings and the 1/4 inch markings are longer than 1/8 inch markings and the 1/8 inch markings are longer than 1/16 inch markings etc.
    It’s easier to count down to the marking that represents the final measurement, with the inch scale.
    However on the centimeter, millimeter wooden ruler, notice how the millimeter markings are different lengths. This is much easier to read when compared to the markings on the black ruler, but unfortunately most rulers and particularly tapes don’t follow this method of marking.
    I think that the manufactures are missing an opportunity to make their metric tapes more people-friendly. Surely any measuring device should be easy to use.

  4. The logical place to specify the unit would be beside the first positive integer on the scale. So, if the numbers represent centimeters, the labels would be “1 cm” “2” “3” and so forth; if they represent millimeters, the labels would be “10 mm” “20” “30” and so forth.

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