Rule Of Thumb

The rule of thumb is an approximate way to measure or do something based on practical experience instead of exact measurement or science.

The urban myth that is better know for that idiom is the one that claims the rule of thumb was a law that allowed a man to beat his wife so long as the switch (or rod) he used was no thicker than his thumb. However, facts are that it has never been part of English common law that a man may beat his wife with a switch so long as it is no thicker than his thumb. What was part of English common law was that a man could legally chastise his wife in moderation but there is no mention whatsoever as to what that entailed.

Some people claim such a ruling was made in England in 1782 by Judge Sir Fancis Buller (17 March 1746 – 5 June 1800) which earned him the nickname of Judge Thumb after a newspaper published a cartoon that attacked a ruling the judge had made. However none of the records of his rulings even hint at him stating that beating a wife with a switch no thicker than a husband’s thumb was ever made.

The myth was something that first shows up in relation to domestic violence in the mid-1970s thanks to feminist Dorothy Louise Taliaferro “Del” Martin (5 May 1921 – 27 August 2008) who used it in a report on domestic violence that was published in 1976. She mentioned a husband’s right to whip his wife in English common law and went on to say this was allowed so long as the switch used was no bigger than his thumb. She then wrote:

… a rule of thumb, so to speak.

The following year, Feminist Terry Davidson further pushed the incorrect concept that a Rule of Thumb existed in English common law which gave rise to the expression.

That’s how the myth got started, and as with many myths, once ingrained in people’s minds, it’s hard for that myth (even once it is debunked) to die.

Idiomation found this particularly intriguing and while researching the idiom, a great many twists and turns were taken to get to the earliest published version of this idiom.

In the 1987 book by James H. Konkel, “Rule-of-thumb Cost Estimating for Building Mechanical Systems” published by McGraw-Hill, it’s obvious the term has nothing to do with anything but estimating, rough measurements, and approximations.

And in the 1969 book “Scientific or Rule-of-thumb Techniqiues of Ground-water Management” by Charles Lee McGuinness, the repeated use of the word rule-of-thumb as a measurement is obvious as the writer speaks of rule-of-thumb decisions and rule-of-thumb judgements and rule-of-thumb evaluations.

Even the 1921 book “Forest Mensuration” by Herman Haupt Chapman states on page 251:

A rule of thumb represents an attempt to formulate a simple rule which can be memorized and by the use of which the contents of trees of any diameter and height may be found.

Mr. Chapman goes on to state that the rules of thumb must be based on either the cubic or board-foot unit, and he provides examples of where to use these different rules and the reasons for doing so. What is interesting is that he also states this:

Both of these rules of thumb are good only for trees of a given height and form factor. They are similar to the European rule of thumb — volume in cube meters equals the diameter squared divided by 1,000.

Further on, he writes:

A more scientific application of a universal rule of thumb is that devised by F.R. Mason (Ref. Rules of Thumb for Volume Determination, Forestry Quarterly, Vol. XIII, 1915, p. 333).

What Idiomation did uncover is that in German there is a similar phrase with regards to the rough approximation which is pi mal daumen which means pi multiplied by the thumb (pi being, of course, 3.14).

It is highly unlike that the German expression referred to making rough measurements while the English expression referred to wife beating. But stranger things have been uncovered while researching idioms, and so Idiomation continued the search.

In the 17 October 1857 edition of Notes and Queries, Thomas Boys wrote about the idiom and how it was also known as the Rule-o-er-thoum and rule o’ the thumb. He mentioned the use of the idiom as meaning an approximation back in 1814 as used in Bordeaux (France). The author also referred to an earlier discussion in Notes and Queries, and indeed, one was found in the 22 August 1857 edition wherein a Mr. H. Draper of Dublin (Ireland) wrote in part:

The origin of this phrase, as applied to anything made or compounded without a precise formula, is to be found in Yorkshire.

In Yorkshire (England)? Was this claim true? In Chambers’s Edinburgh Journal of 1841 on page 172, the following is found in an essay about the rule of thumb.

A more commendable employment of this member in past time was that which gave origin to the phrase, “Rule of thumb.” It was once a common enough practice in Scotland to measure objects in a rough way by calculating a thumb’s-breadth as an inch. Of course, no great accuracy could attend this sort of mensuration, and our more precise times apply the phrase jestingly to every case of rude or careless computation.

John Mactaggart has something to say about the rule of thumb in his 1824 tome, “The Scottish Gallovidian Encyclopedia or The Original, Antiquated, and Natural Curiosities of the South Scotland.”

Rule O’Thumm – Rule of Thumb, the king of all rules. The rule of three, and Pythagorus’s golden rule are nothing to this; it is that rule whereby a person does something which no other can. The Burns wrote Tam O’Shanter by the rule o’thumm; this is the rule of genius, or the rule of nature, which surpasses all the rules of art; every soul knows less or more of this rule, and yet no two know exactly the same.

Even John Jamieson’s “Etymological Dictionary of the Scottish Language” published in 1825 confirms that Rule of Thumm means to do work nearly in the way of guess-work, or at hap-hazard at the very least.

No wife beating references other than the feminist references of the mid-1970s had been found as Idiomation continued going back in time. Perhaps we would find the wife beating reference in earlier references.

In the Francis Grose (11 June 1731 – 12 June 1791) book, “A Classical Dictionary of the Vulgar Tongue” published in 1785, he explained that rule of thumb was something “cone by dint of practice and referred to is as a way to kiss one’s thumb instead of the book.”

We found “A Voyage to the Coast of Africa in 1758 Containing a Succinct Account of the Expedition to, and the Taking of the Island of Goree” by the Reverend John Lindsay  (1686 – 1768), Chaplain of his Majesty’s Ship Fougueux published in Volume 8 of “The Critical Review or Annals of Literature” and dated 1759. The story spoke of the bungling manner in which the charts of a particular well-known port had been laid down. At one point the author wrote:

We should be glad, however to learn how a man is to be landed on the banks of eternity, which is metaphorically an ocean without bounds. Nor are we less curious to be acquainted with the ancient rule of thumb, by which it seems, the charts of Cork harbour have nitherto been finished, though in a bumbling manner.

Surely if any wife beating was involved, the Reverend would have made mention of it, but alas, he did not. Instead he wrote about the confusing manner in which Cork’s harbor had been mapped out. If you can’t trust a Reverend, who can you trust?

In 1692, Sir William Hope, First Baronet of Balcomie (15 April 1660 – 1 February 1724) published the second edition of a book he wrote titled, “The Compleat Fencing-Master in Which is Fully Described the Whole Guards, Parades, and Lessons Belonging to the Small-Sword.” It also included the rules for playing against either artists or others, with blunts as well as with sharps, and as a bonus added feature, there was a section with directions on how to behave in a single combat on horseback.

I know very well that those who understand this Art will be of my opinion, because they know that the Judging of Distance exactly is one of the hardest things to be acquired in all the Art of the small-Sword; and when once it is acquired it is one of the usefulest things, and a Man’s Art as much as a lesson in it, but I am no Man’s retiring too much, unless upon a very good Design, and that hardly any Ignorant of this Art can have, because he doth (as the common Prover is) he doth by rule of Thumb, and not by Art.

And by the fact that the idiom is a common Proverb, that means the roots of this idiom are not found in English common law at all.

INTERESTING SIDE NOTE 1: The second edition of this book was a re-issue of his 1687 book which was originally titled, “The Scots Fencing-Master.” It was also the first book on this topic to be published in Britain.

INTERESTING SIDE NOTE 1.1: He was the younger brother of the First Earl of Hopetoun. He was made a Baronet in March 1698, first of Grantoun, then of Kirkliston, and in 1705, of Balcomie in Fifeshire. Because of his service in the army, he was also made the Deputy-Governor of the castle of Edinburgh.

The earliest known use of it in print appears in a sermon given by the English puritan James Durham  (1622 – 25 June 1658) and printed in “Heaven Upon Earth” in 1658:

Many professed Christians are like to foolish builders, who build by guess, and by rule of thumb and not by Square and Rule.

INTERESTING SIDE NOTE 2: In 1640, a book titled, “Witt’s Recreations – Augmented with Ingenious Conceites for the Wittie and Merrie Medicines for the Melancholic” contained this rhyme spoke of a rule of leg or a rule of foot, but not a rule of thumb.

If Hercules tall stature might be guess’d
But by his thumb, the index of the rest,
In due proportion, the best rule that I
Would chuse, to measure Venus beauty by,
Should be her leg and foot.

But the idiom rule of thumb can still be traced back a little further especially in light of the fact that Sir William Hope referred to it as a common Proverb back in 1687.

In the 15th century (which takes us to the 1400s) there was a law in place in Scotland that referred to a unit of measure using the Latin word pollex which means thumb. It was particularly important when creating statues of monarchs such as Robert III of Scotland (1390 – 1406) The law read thusly:

Thumbs are to be measured by the thumbs of three men, namely one large, one medium, and one small, and should stand in accordance with the medium thumb, or in accordance with the length of three grains of barley without tails.

That’s a pretty specific description albeit still not a specific definition of what consitutes a thumb as a unit of measure. Later, Randle Cotgrave wrote in his French-English Dictionary of 1611 that an inch measure was the breadth of a thumb.

To this end, the rule of thumb concept was around in the early 1400s, and it was an accepted general measure at that point in time. It also appears to have originated in Scotland hence the reference to this idiom being a Scottish proverb.

Idiomation therefore pegs this idiom to some time in the early 1400s and reminds people that the idiom has nothing to do with beating your spouse with a switch or rod that is no thicker than the beater’s thumb.