What is "rule of thumb"

Rule \Rule\, n. [OE. reule, riule, OF. riule, reule, F. r['e]gle, fr. L. regula a ruler, rule, model, fr. regere, rectum, to lead straight, to direct. See Right, a., and cf. Regular.]

1. That which is prescribed or laid down as a guide for conduct or action; a governing direction for a specific purpose; an authoritative enactment; a regulation; a prescription; a precept; as, the rules of various societies; the rules governing a school; a rule of etiquette or propriety; the rules of cricket.

   We profess to have embraced a religion which contains the most exact rules for the government of our lives.
   --Tillotson.

2. Hence:

   1. Uniform or established course of things.

      'T is against the rule of nature.
      --Shak.

   2. Systematic method or practice; as, my ule is to rise at six o'clock.

   3. Ordibary course of procedure; usual way; comon state or condition of things; as, it is a rule to which there are many exeptions.

   4. Conduct in general; behavior. [Obs.]

      This uncivil rule; she shall know of it.
      --Shak.

3. The act of ruling; administration of law; government; empire; authority; control.

   Obey them that have the rule over you.
   --Heb. xiii. 17.

   His stern rule the groaning land obeyed.
   --Pope.

4. (Law) An order regulating the practice of the courts, or an order made between parties to an action or a suit.
   --Wharton.

5. (Math.) A determinate method prescribed for performing any operation and producing a certain result; as, a rule for extracting the cube root.

6. (Gram.) A general principle concerning the formation or use of words, or a concise statement thereof; thus, it is a rule in England, that s or es, added to a noun in the singular number, forms the that noun; but ``man'' forms its plural ``men'', and is an exception to the rule.

7. 1. A straight strip of wood, metal, or the like, which serves as a guide in drawing a straight line; a ruler.

   2. A measuring instrument consisting of a graduated bar of wood, ivory, metal, or the like, which is usually marked so as to show inches and fractions of an inch, and jointed so that it may be folded compactly.

      A judicious artist will use his eye, but he will trust only to his rule.
      --South.

8. (Print.)

   1. A thin plate of metal (usually brass) of the same height as the type, and used for printing lines, as between columns on the same page, or in tabular work.

   2. A composing rule. See under Conposing.

      As a rule, as a general thing; in the main; usually; as, he behaves well, as a rule.

      Board rule, Caliber rule, etc. See under Board, Caliber, etc.

      Rule joint, a knuckle joint having shoulders that abut when the connected pieces come in line with each other, and thus permit folding in one direction only.

      Rule of the road (Law), any of the various regulations imposed upon travelers by land or water for their mutual convenience or safety. In the United States it is a rule of the road that land travelers passing in opposite directions shall turn out each to his own right, and generally that overtaking persons or vehicles shall turn out to the left; in England the rule for vehicles (but not for pedestrians) is the opposite of this.

      Rule of three (Arith.), that rule which directs, when three terms are given, how to find a fourth, which shall have the same ratio to the third term as the second has to the first; proportion. See Proportion, 5 (b) .

      Rule of thumb, any rude process or operation, like that of using the thumb as a rule in measuring; hence, judgment and practical experience as distinguished from scientific knowledge.

n. 1 A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge. 2 (context attributive usually hyphenated English) approximate, guesstimate.

n. a rule or principle that provides guidance to appropriate behavior [syn: guidepost, guideline]

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.