Great comment from slashdot on English Units:

A league is about the distance a healthy man can walk on a good road in one hour. A fathom is about the height of a tall man; it is about eighteen hand widths (fingers closed). A US gallon is the volume of eight pounds of water. An imperial gallon (i.e. the UK gallon) is the volume of ten pounds of water.

One interesting thing about weights. The system of dram/ounce/pound is base 16, which makes division by two a practical measuring operation. Take a pound of something readily dividable, divide it into two equal portions (using a balance scale). Then repeat the process four times. The result is one ounce.

This shows the offsetting virtues of traditional units. While they are difficult to calculate with, they are convenient for measuring things – especially when it come to quantifying things for sale.

For example, consider length: 1 inch = approximately the width of a thumb 1 hand = 4 inches = width of a hand with fingers closed 1 ft = 3 hands 1 yard = 3 ft 1 fathom = 2 yards 1 rod = 5.5 yards = length of ox goad 1 chain = 22 yards = 100 links in standard survey chain

1 furlong = 10 chains = distance ox team can plow without rest 1 mile = 880 fathoms

Notice that if you lay out a square field such that an ox team can plow one furrow across then rest, you get a square with sides of exactly one furlong or 660 ft. The area of that field 43,600 square feet, which is nearly exactly one acre (43,560 ft).

For purposes of round measurement (no fractions), such as you would use in commerce, traditional measurement is far more convenient. If I’m buying liquor, the following units exhaust all the practical measures to which I might wish to round a purchase:

1 mouthful 1 jigger (aka 1 fluid ounce) = 2 mouthfuls 1 jack = 2 jiggers 1 gill = 2 jacks = 4 jiggers 1 cup = 2 gills = 8 jiggers = 16 mouthfuls 1 pint = 2 cups 1 quart = 2 pints = 4 cups 1 gallon = 4 quarts = 8 pints = 16 cups 1 cask = 16 gallons 1 barrel = 2 casks 1 hogshead = 2 barrels 1 butt = 2 hogsheads = 4 barrels 1 tun = 2 butts = 4 hogsheads = 8 barrels

In such a system of measurement, you never, ever have to deal with fractions. Breaking down into smaller units is simply a matter of dividing a whole into two equal parts. So if you want to buy things without having to specify fractions, traditional units are the bee’s knees (equal to 1 / 128 of an inch … no just kidding). That’s not so important in a world with calculators – you just calculate a unit price.

Still, if you want to buy eight feet, three inches of rope, you can measure out twenty-four hands and three thumbs and come rather close.