A rule of thumb is a principle with broad application that is not intended to be strictly
accurate or reliable for every situation.
It is an easily learned and easily applied procedure for approximately calculating or
recalling some value, or for making some determination.
Compare this to heuristic, a similar concept used in mathematical discourse, psychology,
and computer science, particularly in algorithm design.
Origin of the phrase The exact origin of the phrase is uncertain.
The earliest citation comes from J. Durham’s Heaven upon Earth, 1685, ii. 217: "Many profest
Christians are like to foolish builders, who build by guess, and by rule of thumb."
The phrase also exists in other languages, for example Swedish tumregel, Norwegian and
Danish tommelfingerregel, sometimes in the variant "rule of fist", for example Finnish
nyrkkisääntö, Estonian rusikareegel, German Faustregel, Hungarian ökölszabály or Dutch
vuistregel, as well as in Turkish parmak hesabı, and in Hebrew "חוק אצבע" and in Persian
"قاعده سرانگشتی," which is translated as finger tip's rule.
This suggests that it has some antiquity, and does not originate in specifically Germanic
language culture.
Thumb as measurement device The term is thought to originate with carpenters
who used the width of their thumbs rather than rulers for measuring things, cementing
its modern use as an imprecise yet reliable and convenient standard.
This sense of thumb as a unit of measure also appears in Dutch, in which the word for thumb,
duim, also means inch.
The use of a single word or cognate for "inch" and "thumb" is common in many Indo-European
languages, for example, French: pouce inch/thumb; Italian: pollice inch/thumb; Spanish: pulgada
inch, pulgar thumb; Portuguese: polegada inch, polegar thumb; Swedish: tum inch, tumme thumb;
Sanskrit: angulam inch, anguli finger; Slovak: palec, Slovene: palec inch/thumb, Czech: palec
inch/thumb, Hungarian: hüvelyk inch/thumb.
Also in some other languages such as Thai: nîw inch/finger.
Another possible origin of the phrase comes from measurement, in particular in agricultural
fields.
The plants need a fairly precise depth to seed properly, whether planted from seed or
being replanted, but the depth can sometimes be estimated using the thumb.
That is, a "rule of thumb".
According to Gary Martin, "The origin of the phrase remains unknown.
It is likely that it refers to one of the numerous ways that thumbs have been used to
estimate things—judging the alignment or distance of an object by holding the thumb
in one's eye-line, the temperature of brews of beer, measurement of an inch from the joint
to the nail to the tip, or across the thumb, etc.
The phrase joins the whole nine yards as one that probably derives from some form of measurement
but which is unlikely ever to be definitively pinned down."
Thumb used for abuse
It is often claimed that the term's etymological origin lies in a law that limited the maximum
thickness of a stick with which it was permissible for a man to beat his wife.
English common law before the reign of Charles II permitted a man to give his wife "moderate
correction", but no "rule of thumb" has ever been the law in England.
Such "moderate correction" specifically excluded beatings, allowing the husband only to confine
a wife to the household.
Nonetheless, belief in the existence of a "rule of thumb" law to excuse spousal abuse
can be traced as far back as 1782, the year that James Gillray published his satirical
cartoon Judge Thumb.
The cartoon lambastes Sir Francis Buller, an English judge, for allegedly ruling that
a man may legally beat his wife, provided that he used a stick no thicker than his thumb,
although it is questionable whether Buller ever made such a pronouncement.
The Body of Liberties adopted in 1641 by the Massachusetts Bay colonists states, “Every
married woman shall be free from bodily correction or stripes by her husband, unless it be in
his own defense from her assault.”
In the United States, legal decisions in Mississippi and North Carolina make reference to—and
reject—an unnamed "old doctrine" or "ancient law" by which a man was allowed to beat his
wife with a stick no wider than his thumb.
For example, the 1874 case State v. Oliver states: "We assume that the old doctrine that
a husband had the right to whip his wife, provided that he used a switch no larger than
his thumb, is not the law in North Carolina."
In 1976, feminist Del Martin used the phrase "rule of thumb" as a metaphorical reference
to describe such a doctrine.
She was misinterpreted by many as claiming the doctrine as a direct origin of the phrase
and the connection gained currency in 1982, when the U.S. Commission on Civil Rights issued
a report on wife abuse, titled "Under the Rule of Thumb".
Examples of usage Statistical - Rule of 72: A rule of thumb
for exponential growth at a constant rate.
An approximation of the doubling time formula used in population growth, according to which
the doubling time is roughly equal to 70 divided by the percent growth rate.
In terms of money, since most people use the annual effective interest rate for interest
rates between 4% and 12%, the number that gives the most accurate result is actually
72.
Therefore, one may divide 72 by the percent interest rate to determine the approximate
amount of time it would take to double one's money in an investment.
For example, at 8% interest, the investment will double in approximately 9 years.
Musical: Joseph MacDonald, in his book Compleat Theory of the Scots Highland Bagpipe, wrote:
The first Composers of Pipe Music having never heard of any other Instrument or known any
of the Rules ever invented of Musick ... it may not be improper to discover the general
rule by which they Taught & regulated their Time but only their Ear to which they must
only trust.
This Rule we may more properly Call The Rule of Thumb.
In effect it is Much the same, for it was by the four Fingers of the Left hand that
all their Time was measurd & regulated E.G An Adagio in Common Time of Such a Style must
not exceed or fall short [of] Such a number of Fingers, otherwise it was not regular.
If the March was to be but a short Composition, the Ground must be of So many fingers; for
Bars they had no notion of; if a Gathering, commonly of Such a Number, If a Lament, If
a March, & c. according to the Occasion it must Consist of Such a Number.
Tailors' Rule of Thumb: This is the fictional rule described by Jonathan Swift in his satirical
novel Gulliver's Travels:
Then they measured my right Thumb, and desired no more; for by a mathematical Computation,
that twice round the Thumb is once around the Wrist, and so on to the Neck and Waist,
and by the help of my old Shirt, which I displayed on the Ground before them for a Pattern, they
fitted me exactly.
Oersted's rule: Hold right hand with the fingertips in the direction of current.
The line shall be between the magnet and the palm.
Magnet north pole will then turn to the thumb side.
Named for Hans Christian Ørsted, a Danish physicist and chemist who discovered that
electric currents create magnetic fields, an important aspect of electromagnetism.
See also Heuristic argument
Common sense Back-of-the-envelope calculation
Modulor References
External links Every rule of thumb on earth in one place
- A user participation reference project based on Tom Parker's 3 best-selling books and more
than 25 years of research.
A collection of various Rules of Thumb - A wiki based collection of user submitted rules
of thumb Straight Dope - Discussion of the origins
of the term by Cecil Adams.
Business Rules of Thumb . com - A collection of business-oriented rules of thumb
Debunker.com - Article debunking the "Rule of Thumb" connection to wife-beating
European Men Profeminist Network - Faulty legal citations of the bogus wife-beating
law.