Consider the simple process of measuring from here to there, and it becomes clear that our need to make things fit has been passed down to us by careful craftspeople over millennia. Virtually all the units with which we measure our world are derived from…us. They’re anthropomorphic, the measure of man.
A grand pharaoh, perhaps Ramses II, the builder, probably fixed the basic units of Egyptian measurement: the cubit (fingertip to elbow); the palm (breadth of palm); the foot (one dog). We know these measures were fixed because we’ve found masons’ and carpenters’ rulers 3,000 years old, and because the pyramids were constructed with bewilderingly precise cubit dimensions that delight mathematicians and numerologists with their cabalistic implications and harmonies.
Consistency across distance brings up a historical point: Did it matter that the pharaonic cubit in Aswan was identical to the cubit in Thebes? If engineering accuracy demanded common measures, those lengths became strict official standards—often iron bars approved by authorities. Roman engineers depended on consistency across a kingdom and produced official copies of the regula—a wooden stick about 11.625″ long, divided into 12 unciae (hence, our “inch”), and digiti, or finger widths.
Balkanization—the fragmenting of a large state into smaller local holdings—was a defining characteristic of the Middle Ages after Emperor Constantine abandoned Rome as his capital and moved to Constantinople (324 A.D.). Reliance on central Rome’s standards waned, and carpenters in one burg conferred with those in the next burg on a common foot rule. Builders knew a foot was divided into 12 inches (12 was a convenient number divisible by 2, 3, 4, and 6) but where was their iron standard? The inch was traditionally defined as three dry barleycorns place end-to-end. In the 16th century, in Germany, Jacob Köbel offered guidance on determining the length of a “rood” by lining up the feet of 16 men, end to end. His measurements were not identical to those at Constantinople or Thebes, but that, back then, was unimportant.
Mediaeval carpenters used local measures intelligently. What were their standards? Area guilds likely agreed on measures and kept a marked iron prototype from which craftsmen from several specialties made their own measuring and truing tools. They used a plumb bob and triangle as a reliable level. Houses could be laid out accurately using a coil of waxed cord with knots evenly spaced at foot marks. A taut triangle with sides of 3, 4, and 5 (or multiplied products) inevitably makes a right triangle .
The late Middle Ages promoted guarded cooperation among bellicose kingdoms and developed some standardization. Tradition has it that Henry I of England and Wales offered the distance between his long nose and his outstretched fingers as a yard, and that distance was doubtlessly marked on metal and kept in the guildhall treasury, divided geometrically into three feet in a yard and 12 inches in a foot.
Land measure in agrarian society was primary. The Roman mille was based on a thousand strides (two steps), totaling 5,280 feet. An antique measure, seldom used today, is the league of three miles—what you could expect to walk in an hour, stepping briskly. At sea, a league is three nautical miles, something Jules Verne recognized in Twenty Thousand Leagues Under the Sea, the long voyage of Captain Nemo’s NAUTILUS. Within the mile, farmers carved the furlong (“long furrow”), the distance a yoke of oxen could plough before needing a break. The furlong is a singularly agrarian measure and doesn’t loom large for us unless we play the ponies; horse-racing courses are still laid out in furlongs, eight to a mile. Each furlong constituted 10 chains; these were specifically surveyor’s measures based on Gunter’s Chain—a wire chain of 100 links with brass rings at the ends—developed in 1620 by English mathematician and clergyman Edmund Gunter. Young George Washington, surveying western Virginia, used such a chain. A plot measuring one chain in width and 10 chains in length (or any shape of 10 square chains) is an acre. Long land lengths are measured today by a thin (often fiberglass) tape on a reel. Anglophiles may note that one chain is the distance between stumps on a cricket pitch.
The base-10 simplicity of metric measurement is a product of the Age of Enlightenment and the French Revolution. The United States will almost certainly join the rest of the world in metricizing by and by. We suspect, however, that boatwrights may be the last multifractional holdouts. Many tape measures offer both inches and centimeters, which might be useful but is largely clutter. Choose the markings and width you want. Note that not all rulers are marked identically. Usually you will find feet and inches, 16ths, and perhaps even 32nds. Most tapes are marked with squares or diamonds at 16” intervals for stud framing. Some contractors’ tapes have 19.2” intervals for floor joists and roof trusses. Special tapes can be ordered with inches marked in 10ths. Pi (π) tapes directly read pipe diameters from their circumferences. There are also special tapes for concrete and rebar work and for conduit.
HUMBLE FARMERS seldom invested in an expensive Gunter’s chain but did plenty of layout work with a folding iron or wooden rod of 16½ feet, ¼ of a chain (320 rods in a mile). Eighty percent of our population is now urban. We visualize distance along streets. Country people conceive distance along fences, and one rod is an ideal distance between fenceposts.
The industrial revolution demanded consistency and standardization. An inch in Luxembourg must be the same measure as the inch in Rome. The fit of mass-produced products depends on fidelity. You probably measure feet and inches along a tension-rolled metal tape measure with a cupped cross section that stabilizes the tape in midair; broader tape measures are more stable. There will be times, however, when stability is a detriment; a cloth sewing tape of 3’ follows convex and concave surfaces accurately.
The industrial standard before the ubiquitous tape measure was the folding rule, a cleverly constructed flip-flop tool of, usually, 6′. A few craftspeople still prefer the folding rule for its stability at length, and for a few tricks. Pipe fitters take the angle of pipe to be installed by folding a right angle with two rule sections, following the needed angle with the hypotenuse, then noting the inches and fractions where the hypotenuse crosses the baseline. Recreate the triangle on the worktable with the same dimensions.
A WIZARD is a craftsperson adept at the advantages of the seemingly mute framer’s square. There are too many mysteries to relate here. Yes, you can mark an accurate right angle with the square, as you might with the combination square, but the framer’s square far exceeds this ability as a geometric calculator. Using the dimension numbers and the several tables on both sides, the power user can figure the hypotenuse of braces, construct any size of octagon, calculate board feet, figure and mark stair rises and runs, and chart rafter placement. If you are not party to the framing square accomplishments, be careful when you use it to measure: the body (wider leg) and tongue (narrower) are calibrated variously with inches expressed in 16ths, 12ths, and 10ths. Some squares even have a minutely incised table in 100ths so that an exact measurement can be transferred with calipers.
Plain measurement of length is necessary, established, and limited. We can agree that it is not, however, the fine delineation of intricate joinery, dovetailing, or shipboard cabinetry (where the curving hull has nearly no right angles). In this work, one part is measured against another, not written as a dimension.
There are measuring tools that will extend your accuracy, like a few antique dividers and compasses, which can consistently reproduce dimensions more precisely than your tape measure.
Unusual tools from old wood-butcher’s shelves can become standbys: the Stanley Odd Job No. 1 is a ridiculous looking tool that has helped me in more situations than I could have expected. A micrometer caliper like this Brown & Sharp has more accuracy than you’ll need, but its ability to record interior, exterior, and depth measures and to lock them down is valuable. The B&S is pricey, but you can sacrifice a few 1,000ths of accuracy and score a pressed steel caliper for under $10.
It’s doubtful that the Japanese woodcrafters we admire create their extravagant pegged scarf joints from a column of measurements. Measuring is only the first stage of accuracy. What you read on your tape measure and what the mediaeval home builders read from their waxed cords is merely the first step to making a fit. 
Jan Adkins is a regular contributor to WoodenBoat. Visit his blog, Dockwolloping, for more insightful illustrated discussion of all sorts of maritime matters.
In the next issue, we’ll continue this topic with a discussion of current-day best practices of measuring and marking wood before cutting.