If you don’t know an olf from an okta, you needn’t feel even one slug short of a slinch. Help is only mickeys away.

All these terms are units of measurement, distant cousins to familiar words like “inch,” “pound,” and “acre.” For example, an olf (think “olfactory”) is a measure of indoor odor intensity, and an okta (from the Greek word for “eight”) is a measure of cloud cover. A slug is a measure of mass acceleration. NASA engineers know that there are 12 slugs to the slinch, even if both units are losing ground to their metric counterparts. A mickey is the smallest visible unit of cursor motion your mouse (hence the name) can produce on a computer screen.

These definitions and hundreds more appear on a website created by Russell J. Rowlett. This is one of several Internet sites devoted to measurement, but the scope, readability, and rigor on this site are exceptional. Rowlett includes some archaic units (the biblical “talent,” for example), but most of the terms he cites have a present-day use.

The number and variety of terms are far more vast than you might expect, thanks largely to the unique requirements of science and commerce. For example, you’ll find almost 30 entries under “degree” alone. You already know about measuring temperature in degrees Fahrenheit or degrees Celsius. But what about degrees MacMichael, which measure the viscosity of chocolate, and degrees Quevenne, which measure the density of milk? (When comparing your milk to some other liquid, just remember that five degrees Quevenne equal one degree Twaddle.)

Rowlett’s site began as a series of notes for his classes. He later decided the notes would be more useful online in a searchable format. Expanded by suggestions from around the world, the site has evolved into more than a series of lists. Rowlett’s comments clarify why old units like rods and furlongs (measures of distance that still appear in old real estate records) fade out of use, and why odd-sounding units like olf, oktas, and mickeys come into being.

The basic concept suggests that whether we’re weighing cabbages, tracking time on the job, or telling someone the distance to an airport, we need specific words suitable for each task. Weight and time, for example, are incommensurate, meaning that they can’t be measured in the same way. It makes no sense to say that a cabbage weighs “ten minutes.” Still more words are needed to convey differences in scale. No one wants to know the distance to an airport in inches.

That much is logical enough, perhaps even obvious. It doesn’t explain, however, why we buy whiskey by the fifth, drink beer by the pint, and measure out rum by the jigger. These liquid measures are in principle commensurate, so it’s possible to express any of them in liters or in terms of each other. But ask a bartender for 10.667 jiggers of beer instead of a pint, and you’re apt to be told that you’ve already had enough.

Former U.S. Supreme Court Justice Oliver Wendell Holmes, Jr., once wrote: “A page of history is worth a volume of logic.” Holmes was talking about law, but his remark helps to explain why people create, and continue to use, so many different units of measurement. The definitions at Rowlett’s web site reflect a continuing tension between convenience and precision or, if you will, between logic and history.

Despite all the efforts at standard-ization,” Rowlett says, “ad hoc units are created on all sides, all of the time. And they keep coming up. There are as many new ones as there are old ones. The more we standardize, it seems like the more we also have our own little set of ad hoc units that we like to use.”

Most of our traditional units began with ad hoc comparisons to items readily at hand. “Foot” is an obvious example. A “rod” would once have referred to an actual rod or staff. Forty rods make a furlong, which comes from an old form of “furrow-long,” the traditional length of an Anglo-Saxon furrow. At various times, units like these were standardized, first by custom and then by royal edict or the formal blessing of some official commission. Commerce across national boundaries naturally expanded the need for additional definitions.

Specialized crafts and trades tend to keep using their own metric conventions. Fabric manufacturers still sell cloth by the bolt, and sawmills can compute the volume of neatly stacked lumber in board-feet (a unit 12 inches square and one inch deep), no matter how thick individual boards may be. Time-and-motion experts use a measure known as a therblig, a variable backward spelling of the last name of a husband-and-wife team of industrial psychologists, the Gilbreths. Writers buy paper by the ream, a term based on an Arabic word for “bundle.”

In everyday life, the rambling growth of ad hoc units doesn’t bother us. Americans, as well as the British, are used to thinking of distance on a scale that has 12 inches to a foot, three feet to a yard, and 5,280 feet to the mile. However, traditional measures like this make scientific and technical calculations (not to mention eighth-grade homework) unnecessarily difficult, especially with numbers on very large and very small scales. It’s a great boon to math and memory if new units of scale can be derived simply by multiplying by ten. And if you’re doing calculations across fields (combining pressure, temperature, and time), you appreciate metrical logic.

The idea that the units in one field should be the same as those in another,” Rowlett says, “or that everything should be related to another, that’s one of the ideas of the Enlightenment that led to the creation of the metric system.”

Rowlett points out that the United States has, as a matter of international law, accepted the metric system since 1875, when this nation was one of the original signatories of a treaty signed in Paris. The metric system not only standardized terminology on a 10-based scale (like milli-, centi-, and kilo-), but it also defined how incommensurable units should be related to each other.

The International Systems of Units (SI, for short, based on its first two initials of the French Système International d’Unités) is a triumph of minimalist logic. All weights and measures, whether metric or otherwise, are now linked directly or indirectly through the SI. It has only seven base units, meaning that they are not derived by reference to other units.

The technocrats who maintain the purity of the SI have approved an additional 22 “derived” SI units, all but a few named after scientists. Some are familiar, like the watt and volt. Others, like the henry (a measure of magnetism) and the sievert (radiation dosage) are likely to be used only by specialists.

The SI technocrats redefine even a base unit whenever its historical basis is found to be insufficiently precise. For example, scientists concluded that the small unit of time familiar to us all, the “second,” could not be defined as a fraction of a solar day because solar days vary minutely in duration. An SI second is now defined by reference to the decay of a cesium atom.

The rest of us — especially Americans — continue to mix and match units in cheerful defiance of logic. “Caught in a slow-moving transition from customary to metric units,” Rowlett writes on his web page, “we employ a fascinating and sometimes frustrating mixture of units in talking about the same things. We measure the length of a race in meters, but the length of the long jump event in feet and inches. We speak of an engine’s power in horsepower and its displacement in liters…a hurricane’s wind speed in knots and its central pressure in millibars.”

In short, the tension between history and logic won’t be resolved anytime soon, if ever. My computer’s memory is measured in gigabytes and its chip speed in gigahertz. I can watch its cursor creep mickey by mickey across a screen with a resolution that computer engineers have measured twip by twip (short for “twentieth of a point”). But I still buy paper by the ream.

Fred D. Baldwin was a freelance contributor to Endeavors.

Russell Rowlett is director of the Center for Mathematics and Science Education, clinical professor of education, and adjunct professor of mathematics. Visit his measurements web site.