The phrase temperament ordinaire (French tempérament ordinaire, meaning literally "ordinary temperament" or "usual temperament") is a term for musical intonation, particularly the tempered tuning of keyboard instruments. In modern usage, it usually refers to temperaments falling within the range (as understood broadly) of tunings now known as "well-tempered". The expression occurs primarily in French-language works of the 17th and 18th centuries concerning theory and practice of musical intonation with regard to keyboard instruments. It is discussed again, in the same or a similar musical application, in modern literature concerned with historical practices relating to keyboard instruments and performance.
17th-century use and application One of the early historical documents in which the phrase was used is Christiaan Huygens' Lettre touchant le cycle harmonique, ("Letter concerning the harmonic cycle") of 1691. This refers several times, in a comparative way, to "temperament ordinaire". The main purpose of Huygens' letter was to describe and discuss an unconventional 31-fold division of the octave. He did this by first recapitulating a conventional known temperament of his time, and then he compared that with his new scheme (which actually had been approximately conceived before, albeit without Huygens' mathematical precision); and he discussed the differences. Huygens' description of the conventional arrangement was quite precise, and it is clearly identifiable with what is now classified as (quarter-comma) meantone temperament. Huygens referred to this conventional arrangement, variously, in the course of his comparisons, as "the Temperament that I have just explained", "the Temperament", "the ordinary Temperament" (French: temperament ordinaire), "the Ordinary Temperament" (with both words capitalized), and then by mentioning "the new Temperament" as contrasted with "the one that everyone uses". Accordingly, it does appear that for Huygens in 1691, temperament ordinaire was a phrase denoting just the temperament in ordinary use, with no sign that he was using this expression as a proper or conventional name or label. It appears from his description that Huygens believed that the ordinary tuning method then in common use was (quarter-comma) meantone temperament.
18th century The term was later used in the Encyclopédie of Diderot and D'Alembert, published in Paris in 1751–1772, which contains an article on temperament written by Jean-Jacques Rousseau. The article discusses the contrasting merits of equal temperament and of an arrangement referred to as "temperament ordinaire", "temperament" (without qualifier), and also as "the common rule of temperament", and gives practical instructions how to tune a keyboard to this temperament. In regard to the use of the expressions denoting the temperament in this article, it is noticeable that while all occurrences of the word temperament in the original article stand italicized, the accompanying words, including ordinaire, never are. That seems to show that Rousseau was using the phrase just to denote descriptively what he regarded as usual, rather than as a proper name or conventional designatory label. As for the nature and identity of the temperament that Rousseau called the common one, the content of the article (see instructions reproduced below) leaves it clear that it was a circulating temperament, rather than the quarter-comma meantone referred to by Huygens about 60 years previously. It is relevant to reproduce here the actual instructions from the Encyclopedie (in translation) for tuning to "the common rule of temperament", so as to leave it clear where they are specific, and where they are vague:
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