Multiple Rulers That Do Not Rule Them All

A ruler is a strip of wood, metal or other material marked off in inches or millimeters, used for drawing lines, measuring etc. This is a slight paraphrase of a dictionary definition of a ruler. It seems straightforward, but my experience is that many more alternatives exist than I ever expected could exist.

My father sent me an image of an old ruler, probably from the 1970s, that he ran across. The email subject was “no value” as he saw it as not particularly interesting. When I took a look at it, I was surprised, as it is the first common ruler I’ve seen that clearly and explicitly labels millimeters and centimeters. You may recall my essay The American “Metric” Ruler where I show how four commonly available US rulers are marked. They are reproduced below:

Essentially the labels for cm, mm, or mm and cm, are placed without any clue as to how they should be interpreted.

The new ruler shows how subtle design can be, and how often it is ignored. The first five millimeters have a label that indicates millimeters, but with capital letters, which are commonly used for some reason. The first numerical label, 1, has CM with the cm rule marks which are the longest in vertical length. One has to note the next longest are 5 mm or 5/10 cm markings, and the shortest are millimeter increments. Next the 10 centimeter graduation is marked. I’ve never seen this explicit an attempt to delineate centimeters and millimeters on a common swag ruler. Here is a full graphic of the ruler:

The ruler also shows a default choice by the designer. Two measurements units are used to describe a single distance. The notion of using two measurement units is ubiquitous in the US. 5′ 10″ for a person’s height is the most obvious example. The parsimony of millimeters only was not even contemplated, I imagine, probably from the inertia of the US measurement mindset. It seems to suggest one should write measured lengths as a mixture of centimeters and millimeters. In other words, 25 millimeters would be written as 20 cm and 5 mm.

My friend Pierre has his “rule of flat surfaces.” When I took machine shop class with Pierre, he pointed out that any open flat surface will immediately be interpreted as a table, and begin accumulating objects. Many times a new flat surface of a machine tool would appear, and soon it would accumulate cans, pens, and sundry objects. The surface would be obscured in no time, and saturated with items. I thought about this when I encountered another swag ruler from a large electronics supplier. Rulers generally have a set of markings, and plenty of open area for the inclusion of advertising, or other information. The new ruler packed every square millimeter with design information in an attempt to have as much utility as possible. I see it as a tour through many of the topics addressed here over the years. If we had become strictly metric in the US, much of what is on this ruler could be eliminated and simplified.

The ruler has two “sides” one side is labeled “MIL RULE” which means it is mil friendly. Here is a full image of the MIL side:

Mil Side – click to enlarge

For those of you who live in refined metric countries such as Australia, or less  refined metric countries, such as the EU, a mil is “defined’ as one-thousandth of an inch. The Olde English units used in the US do not have a unit which is smaller than a barleycorn, which is one- third of an inch. There are 333 mils to a barleycorn. The New English units of the Imperial System use a thou, which is another name for a mil. The graduations along the top of this 12 inch ruler are marked in fractions, that are essentially quarters of an inch. At six inches the rule changes to decimal inches in tenths. The tenths would be 100 mils each, but despite the labeling as a “MIL RULE” the scale is not in mils.

The upper left corner has a quick table for printed circuit board copper thickness. It relates the weight in ounces of a one-square foot patch of copper to thicknesses in MILs, and more appropriately, in micrometers. I addressed this strange manner of expressing copper thickness in my essay The Cuprous Proxy. The defining gauge for copper is given as a mass that is then related to a thickness. I don’t recall a single electrical engineer that has ever expressed a need to determine the mass of copper on a PCB. Gauge, is a vacuous and meaningless identification as I discuss in Don’t Get Engaged With Gage.

Just below the copper thickness table are Metric fastener diameters. They are easy, M6 means 6 mm, and M5 is 5 mm, and so on. The equivalent diameter in MILs are presented as 252, 209 and so on. At best MILS are argot, and if we had embraced metric, they would not be clogging the arteries of our commerce. Below the metric fastener sizes, are US “standard” machine screw holes. They range from #2 to #10 gauge sizes, to 1/4″ for the largest. The designation discontinuity assaults a rational mind, but seems logical to denizens of US manufacturing. Below each US machine screw hole are values in MILs. The uncorrelated values we call “standard” are explored in my essay Without Metric Threads We’re Screwed.

A number of font heights are offered to show how large different values in MILs are. Test points (used to measure electronic devices during manufacture) with diameters that are 30, 40, 60 and 80 MIL are presented.

Then stranded wire hole references are given. They are in a gauge value which is AWG or American Wire Gauge, which is not a dimension, only a nationalistic proxy. As the gauge number goes up, the physical dimension of the wires go down. I’ve discussed this mess in The Metric Mess is Hard Wired in the US.

MM side – click to enlarge

Well, with that horror show over, we can now look at the metric side, which despite its designation as MM RULE, has centimeter markings. There are millimeters and centimeters, or centimeters and tenths of centimeters, but only centimeters are marked explicitly. Clearly the designers of this reference rule don’t have a good command of what a millimeter rule is, or it would go from 10 to 300.

I’m not expert enough to go into the intricacies of integrated circuit pads on the left, but I have discussed the embarrassment that is Olde English designations of surface mount devices in my essay US Electronics: a Metric Peg in an Imperial Hole. They offer a conversion table that shows an 0603 “English” and 0603 metric (which is the world standard) are not the same size. This is true for 0402, whose metric designation is not on the list. The accepted international standard sizes are in parentheses, whereas the feral designations that have no standards backing are placed first. 

This ruler is an embodiment of the contemporary ossified measurement set in the US. Americans have the false sense they are great innovators, but when one looks at their measurement system, it is completely unjustified in any utilitarian or rational sense, the evidence is against it. The only positive thought I have is the ruler is a PCB with red soldermask and white component legends. Red soldermask usually designates a work in progress, it is changed to green when the work is ready for production. I’ll hope I see a green ruler someday.


The well-known guitarist, Jimi Hendrix, was a science fiction enthusiast. His song Purple Haze was inspired by the Philip Jose Farmer book Night of Light. I read Farmer’s book Time’s Last Gift in high school, which I though was rather good. I decided with my life on pandemic time, that I would read Night of Light. Here is the quotation that establishes the provenance:

Above it, the moon shown golden-purple in the center and silver-purple around the edges. So huge was it, it seemed to be falling, and this apparent down-hurtling was strengthened by the sight shifting hue in the purple haze.

The book was written in 1957, and while the beginning was compelling, for me, the second half was not. So why this postscript? Because Philip Jose Farmer used nothing but metric units in his book, and that was quite a surprise. He used metric without exception. Here are a couple of examples:

A stone statue reared toward the ceiling. It was fully sixty meters high, a titanic woman…..

But the work of sawing through flesh and bone left him panting as if he’d run several kilometers.

What did strike me was the anomalous use of metric as it was imagined in 1957:

The gray half-moon of the upper half of the 5.08 centimeter disk became luminous.

Wow, that’s a rather precise number, in contemporary terms it would be 50.8 mm. My only charitable interpretation was that it could be a proverbial size that future inhabitants would recognize, like 5280 feet is now.

He goes on:

Carmondy rotated the movable upper part of the disc, and the face seemed to spring out of the screen, and to hang, much enlarged, 16 centimeters in front of the disc.

Again, that seems rather precise, probably a bit more than an average person might intuit. Of course if that’s the case let’s just go with 160 mm, which is about 1.6 hand-widths.

Later it’s a bit more natural when he states: “…and stopped only a few centimeters from Carmondy’s.” Sadly, in the science fiction future of 1957, millimeters are apparently not a thing. They appear nowhere in the book. For me, that’s a sad and apocalyptic future.

An example that struck me as the author trying to imagine the future use of the metric system, and stuffing it into contemporary Olde English usage, is:

His room was nearly two hectometers down the broad stone-walled corridor.

Wow, I had to read that twice as I thought it said hectares at first. No, in the future, instead of saying 200 meters, it will be easier to say 2 hectometers.

But lest the reader think I’m making light of this, I’m not. I’m pleased he tried. Predicting the future is what science fiction writers are supposed to do. This usage is but a peccadillo. The real scandal, is that in the US of the future, it is hard to imaging metric usage at all. It is there his efforts as a science fiction prognosticator fail. He was too optimistic.

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