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.

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Related essays:

The Design of Everyday Rulers

Stickin’ it to Yardsticks

The American “Metric Ruler”

America’s Fractional Mind