Rule of Thumb - Examples of Usage

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 (using continuous compounding, the actual number would be about 69.31 or 100 times the natural logarithm of 2). In terms of money, since most people use the annual effective interest rate (which is equivalent to annual compounding) 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 (72/8 = 9).
• Musical: Joseph MacDonald, in his book Compleat Theory of the Scots Highland Bagpipe (ca. 1760), 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 (having neither of Common or Triple Time, Crotchet or Quaver) 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 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 (often rendered Oersted in English; 14 August 1777 – 9 March 1851), a Danish physicist and chemist who discovered that electric currents create magnetic fields, an important aspect of electromagnetism.