The Pseudo-Inch

By The Metric Maven

Bulldog Edition

I have a friend, Pierre, who has a passion for woodworking, but a passion for metric?—not so much. My knowledge of woodworking is at best minimal, but Pierre does his best to enlighten me. One evening I received an email from him after he had watched a woodworking program by Roy Underhill.  Roy is an American, and a bit of legend in woodworking as I understand it. During the program, Roy explained a Swedish method of teaching called sloyd. My friend Pierre then related:

The original Swedish drawings in the book of exercises he used were measured in metric. Underhill says, “I took up woodworking so I wouldn’t have to learn metric.” Thought you’d want to know that.

Next, Underhill was showing how to measure 2 cm over on a board using a Swedish wooden folding rule. He said he got it from some visiting Swedish woodworking friends, because his American one doesn’t have centimeters on the back. He said, they said, and this is really the part I wanted you to know, that in Sweden, his woodworking friends use only inches.

Then, he flipped the Swedish folding rule over, and it showed inches. Here’s another interesting part, I hope you aren’t asleep yet.

The Swedish “inch” is bigger than our inch. Holding the rules one over the other, you could clearly see the Swedish “inch” is about 1/16 bigger.

WTF, man?

Well, the problem is, there is really no such thing as “an inch.”  Why do we think that an inch exists?—well, therein lies the tale. The Wikipedia entry for “the inch” has an enlightening “inch converter” which was used before the age of the metric system. Here is the illustration:

19th Century Inch Converter

19th Century Inch Converter — Wikipedia Commons

One can see that “the inch” has many different lengths in the 19th century. Many of them are considerably different in length. Note the Moscow and Russian inch are not even close to one another. Here’s what I have surmised from this converter about inches:

  • Hamburgh – Inch divided into 8 parts. 1 inch ≈ 23.2 mm
  • Austrian – Inch divided into 8 parts. 1 inch ≈ 25.8 mm
  • Itallian – Inch divided into 8 parts. 1 inch ≈ 28.3 mm
  • Bremen – Inch divided into 10 parts. 1 inch ≈ 23.7 mm
  • Swedish – Inch divided into 12 parts. 1 inch ≈ 24.3 mm
  • Turkish – Inch divided into 12 parts. 1 inch ≈ 31.3 mm
  • Bavarian – Inch divided into 12 parts. 1 inch ≈ 24.0 mm
  • Spanish – Inch divided into 12 parts. 1 inch ≈ 23.0 mm
  • Portuguese – Inch divided into 12 parts. 1 inch ≈ 27.0 mm
  • Moscow – Inch divided into 12 parts. 1 inch ≈ 27.7 mm
  • Russian – Inch divided into 8 parts. 1 inch ≈ 44.1 mm
  • Amsterdam – Inch divided into 12 parts. 1 inch ≈ 23.5 mm
  • Rhynland – Inch divided into 12 parts. 1 inch ≈ 26.1 mm
  • French – Inch divided into 12 parts. 1 inch ≈ 27.0 mm
  • Fr. Metre – Centimetres divided into millimetres
  • English – Inch divided into 32 parts. 1 inch ≈ 25.3 mm
The “inch” is a perfect example of what I call  a retread unit. This is a unit with a name that is used over and over for different quantities, which renders it meaningless as a standard.
This lack of a clear international definition of the inch, produced a strange misunderstanding in the early part of the Twentieth Century between Americans, British and Australians–who were the contractors:
In 1909 the American firm Pratt and Whitney was contracted to supply all the equipment for the Lithgow plant to specifications that  would ensure interchangeability of components with British rifles. But the parts would not fit because no one told the Americans that the British drawings used two different standards of length: dimensions above 2 inches were expressed in inches aligned with the imperial yard; but dimensions below 2 inches were based on the `Enfield inch’, a standard used by the Royal Small Arms Factory at Enfield. (For Good Measure Jan Todd 2004 pg 58)
Here is the history of the Swedish inch according to Wikipedia:
In Sweden, from 1855 to 1863, the existing Swedish “working inch” of ≈24.74 mm was replaced by a “decimal inch” of ≈29.69 mm which was one-tenth of the Swedish foot. Proponents argued that a decimal system would simplify calculations. However, having two different Swedish inch measures (and the English inch on top of that) proved to be complicated. So in a transition period between 1878 and 1889 the metric units were introduced as the overall standard measures. However, the various inches survived some time in building and construction trades.

         click to enlarge image

Originally the Anglo-Saxons used their smallest standard, the barleycorn, to define the inch. We still use barleycorns to define shoe sizes in the US. The standard was three barleycorns in a row make an inch. I call this “The Barleycorn Inch.” The barleycorn is also called the grain. There are 7000 grains to a pound. In the 20th century, the British and Americans decided to define their inch as exactly 25.4 mm. It is sometimes called the “Industrial Inch.” This is what Americans call “The Inch.” This inch was decimalized in many industrial applications and is found on many, many US technical drawings. The decimalization of the inch is the basis for our machine tools, yet finding a ruler in the US with decimalized inches is almost impossible. Our educational system doesn’t even teach decimalized inch units and how they are used by industry. The fascination with fractions in this country is beyond my understanding.

I have converted my engineering work to be entirely metric, which can cause heartburn for some American vendors. Recently I finished a PCB design for a broadband microwave device. I sent a PCB fabricator gerber files and metric drawings of the device to be fabricated. They requested I give them drawings with dimensions in inches—I demurred. They asked again—again I told them no. Their third email to me pointed out that inches were what their equipment was calibrated to, and what they are trained to use, are used in the US, and they wanted inches.

Dimensional Standard for Barleycorn Inches

The PCB files I sent actually were in inches as forced by US industry, but the drawings were not–so why was the vendor so insistent that I provide inch drawings? Well, in Engineering, engineering drawings define the controlling dimension. The controlling dimension is the one to which the manufacturer is expected to conform. If I have a 100 mm drawing dimension, that is what the length is supposed to be, with a tolerance in mm, not 3.937 inches and a tolerance in mils. The conversion from inch to mm is exact, the conversion from mm to inches is inexact. The vendor did not want to be held to the accuracy required in mm when they had inches on their fabrication equipment.

The point I made to them is that the controlling dimension on all engineering drawings in the US is actually metric. This is because we use the Industrial inch in the US. The definition of the US inch is 25.4 mm exactly. All the “inches” of the vendor’s equipment are calibrated and controlled by the meter, so the controlling dimension of all US drawings in inches, is actually in terms of mm (i.e. meters), we just don’t acknowledge this.  The situation ended up resolving itself, and the boards were successfully fabricated. As a country we pretend that we use something called “the inch,” but it is derived from a metric standard. We use metric as our base standard, but do not adopt the convenience of the actual metric system, preferring to pretend we have “our own American/Standard” system. This is delusional.

The power the inch has over US citizens and others appears to have caused early users of metric to impose vacuous imperial conventions on the metric system. You will note that on the “inch converter” from Wikipedia the centimeter also appears in the list. As I have explained in detail in other blogs, the centimeter impedes the ease of use and soils the elegance the metric system offers. The use of millimeters only, allows for a simple and accurate implementation of metric—often without decimal points. This is experienced by Australian construction workers every day. The fact that Roy Underhill doesn’t understand the power of using millimeters, and slavishly uses centimeters instead, makes him less than a legend to me. He simply embraces folklore. The centimeter is the perfect example of an unnecessary division which appears to exist only to preserve an unnecessary and ill-defined unit of magnitude called “the inch.”. The centimeter is but a pseudo-inch, demanded by tradition and not by necessity. Its utility has proven to be non-existant in practice. Remember friends don’t let friends use centimeters. And The inch?—the definite article?—well, it’s much like fairies, and other mythical creatures, it only exists in our imagination.

Updated 2013-01-31

Related essays:

Metamorphosis and Millimeters

Building a Metric Shed

5 thoughts on “The Pseudo-Inch

  1. Interesting that the Russian inch is such an outlier; there must be a story behind it?

    I do have a pretty good decimal tape measure – Stanley 33-272 that I use with wood working. The first 6 inches have a fine marking of 0.02″, the rest drops down to 0.1″ . I had the experience of working with a skilled carpenter ( he also did steel work ) on a project, he saw that tape measure and how I avoided dealing with error prone fractions and wanted one. ( I’ve thought that using metric inch might help people move on to mm… ) . The problem is there are no metric inch tape measures in lengths over 12′ that I’ve found.

    One of the points he made is that most carpentry (until you get to finish molding) is done with 1/8″ accuracy (including steel – where a weld can make up most of an eighth) – which is a bit over 3mm – moving to mm would tighten up the slop a bit and avoid fraction errors at the same time.

    Finding a pure metric tape measure is also not easy. I bought a couple from Australia that with shipping were expensive – and plagued with cm markings. I have come across metric plywood, but for home repair, most material is 4×8′ or the like. ( of course there is the amusing assortment of 2×4 lumber that shrunk over the years and might as well be managed in mm.)

    Having made a few fractional errors on construction projects myself, I wonder how much material is wasted from such errors – it has to be significant.

    One last thought – CAD programs struggle with doing dual units – if the inch had been defined as 25.6mm, ( 2^8/10) many errors would have been avoided. Instead, the systems are forced to have a native unit of nm – which reduces the maximum size of circuit board design with 32bit integer math.

    • For ECAD if you’re doing a PCB over 2m in any dimension you’re going to have more problems than just unit conversions.

    • If you keep your eye on eBay, you might be able to find a mm ruled, metric only tape for cheap. For instance, I got a Stanley 33-637 on there a few years ago for under $15. They aren’t always available though. There were some listed back in December, that sold for $5 + $5 shipping. I wish I had seen that, as I probably would’ve bought a 2nd one.

  2. It wasn’t until 1959, that the US, Canada and the UK adopted the 25.4 mm inch. Before that the USC inch was significantly different from the imperial inch that precision components made to one didn’t fit to the other. The USC foot based on 12 USC inches is still in use today as the US survey foot. The US could not completely discard the old USC inch because the difference was significant.

    Despite the uniformity achieved between the USC inch and the imperial inch after 1960, any legacy components still produced today to the 25.4 mm inch may not fit in products originally made to either the USC inch or the imperial inch.

    And to top it off, there are still those who think USC and imperial can be interchanged. They are not the same, plain and simple.

Comments are closed.