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Bob's rule is a simple modification of the Imperial (feet and inches) system that combines features of the Imperial and Metric systems. By dividing one inch into 24 bobs, an inch is divisible into increments of 1/2", 1/3", 1/4", 1/6", 1/8", 1/12" and 1/24".
Read more about it below. Distributed exclusively by Whitechapel Ltd.
16' Bobsrule Tape Measure
6' Self-Adhesive Bobsrule Tape
What the press has to say about BobsrulePopular Woodworking magazine - June 2005
Water Craft Magazine (UK)
Jackson Hole News & Guide article
Bobsrule: The Rationale
How can the ruler, an age old measuring device for woodworkers, possibly be improved upon? Read on to find out!
The ProblemNeither the familiar system of Feet and Inches nor the Metric system of meters and millimeters are suited to the type of measurement and calculation woodworkers and other craftspeople use day in day out.
The SolutionA system of measurement that retains Feet and Inches and then divides the Inch into 24 parts.
The ArgumentThe Metric system excels at smaller measurement while Feet and Inches are better suited to larger. Because of these innate characteristics I have always used both, often in awkward combination. The Metric system’s only real asset is the particular size of the millimeter. This happens to be an ideal “small” unit of measure. Smaller than a 1/16" and larger than a 1/32", it can be distinguished easily without reading glasses while at the same time is fine enough to be a basic increment of woodworking precision.
In every other respect the Metric system is inadequate. There can be no rational argument in favor of a system that divides and multiplies by tens. It is perhaps not surprising that we break the day into 24 hours rather than 20, the hour into 60 minutes rather than 100 and so on. By the same token we divide the circle into 360 degrees rather than 100 or 1000. It is because the basic building block of twelve is vastly more versatile than ten, whose capacity for easy division by five and multiplication by ten is a talent I cannot remember ever having needed in 25 years of woodworking.
Feet and Inches supply a manageable series of larger increments. Feet and Inches break up what in the Metric system tend to be long easily corrupted numbers into readily recalled chunks. For example a Metric length of 2286mm equals 7 feet 6 inches. While 2286 might easily become 2268 in our fallible memories, it is hard to not notice 7 feet 6 inches becoming 6 feet 7 inches. The inspired division of the foot into 12 inches rather than 10 allows easy division into halves, thirds and quarters.
The great failing of Feet and Inches is the multitude of unsatisfactory fractions that make addition, subtraction and division an exercise in mental agility and provide ample opportunity for error. Nobody can quickly and reliably add 11 7/16" and 4 5/32" then divide the result by two, let alone three.
With our new rulers all these issues become a thing of the past and we find a system that combines the best of both of the old without any of their failings. This is achieved by employing the 1/24" as the basic increment. The 1/24" is a useful increment in a number of ways. Like the foot, which can be easily divided into halves, thirds, quarters, sixths and twelfths, an inch made up of 1/24th’s can be divided into halves, thirds, quarters, sixths, eights, and twelfths (try that with the metric system). The 1/24" is very close in size to the millimeter and so has the same natural advantage as an ideal small unit. These rulers allow us to take advantage of the largely forgotten 1/24" and elevate it to its rightful place as a core unit of measurement.
My hope is that the user of these rulers will forget all about the 1/8", 1/16", 1/32", and 1/64" and come to depend entirely on the 1/24". Once these other fractions are dismissed it will no longer be necessary to think of the 1/24" as a fraction at all. Without wishing to sound vain I would suggest that this unit of measurement be called the “Bob”. This will allow measurements to be remembered in much the same way as British currency was before decimalization, when a train ticket to Oxford might have cost “One pound seven and six” (1 pound, 7 shillings, and 6 pence). For example the width of the desk I’m working at would be “Four foot eight and ten bob” (4 feet, eight inches and ten 1/24ths) and ultimately, once all memory of other fractions have been lost, “four foot eight and ten” both of which are a lot easier to remember and identify than either “four foot eight and seventeen thirty-seconds” or “one thousand four hundred and thirty two millimeters”. Where a greater level of precision is needed the Bob can easily be divided in half by eye and further still by working to one side or the other, rather than the middle of the rule markings (the terms “Fat Bob” and “Thin Bob” come to mind). Even without the confusing profusion of tiny markings found on the standard ruler the woodworker using Bobs can be consistently accurate to about 1/100".
I’m not yet enough of a megalomaniac to think that this revolutionary system of measurement will roll back the tide of metrification that has swept the entire world with the singular exception of our own shores. I seriously doubt it will make noticeable headway against the system of fractions used here. But it may have a good future in the workshop as by far the best way of measuring, marking, remembering and calculating. It is within the confines of that environment that I pin my hopes!
The InstrumentsThe quality of our English made rulers stands head and shoulders above even the fanciest rulers available in the US. Rather that the conventional photo-etching process that results in shallow markings, these rulers are entirely hand engraved using high precision pantographs. These deep graduations are black filled for superior legibility on the background of non-reflective chrome.
These attributes along with our minimalist approach to layout result in a ruler that makes every effort not to confuse. Barring catastrophic accident these hardened steel rulers will last forever.
Creator - Bobsrule