triad

—triadic, adj. —triadism, n.

/truy"ad, -euhd/, n.

1. a group of three, esp. of three closely related persons or things.

2. Chem.

a. an element, atom, or group having a valence of three. Cf. monad (def. 2), dyad (def. 3).

b. a group of three closely related compounds or elements, as isomers or halides.

3. Music. a chord of three tones, esp. one consisting of a given tone with its major or minor third and its perfect, augmented, or diminished fifth.

4. (cap.) Mil. the three categories of strategic-nuclear-weapons delivery systems: bombers, land-based missiles, and missile-firing submarines.

[1540-50; < L triad- (s. of trias) < Gk triás See TRI-, -AD¹]

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Term used variously for secret societies in Qing-dynasty China (and sometimes earlier), for modern Chinese crime gangs, and for crime gangs of other Asian nationals operating in their own countries or abroad.

A secret society with the name Triad started operating in the early 19th century in southern China, where it took root and spread. In the 1850s Triad rebellions threatened Shanghai and Xiamen (Amoy) and contributed to the revolution of 1911. Chinese secret societies have in common the swearing of an oath to join, strict rules, a family relationship among members, the duty of mutual help, a hierarchy of functions, and hereditary membership within families.

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▪ chemistry

      in chemistry, any of several sets of three chemically similar elements, the atomic weight of one of which is approximately equal to the mean of the atomic weights of the other two. Such triads—including chlorine-bromine-iodine, calcium-strontium-barium, and sulfur-selenium-tellurium—were noted by the German chemist J.W. Döbereiner between 1817 and 1829. The triad was the earliest atomic-weight classification of the elements.

▪ music

 in music, a chord made up of three tones, called chord factors, of the diatonic scale: root, third, and fifth. The system of diatonic triads is the basis of tonal harmony in music.

      Triads are classified according to intervals formed above the root. If the factors of the triad are a major third and a perfect fifth above the root, the triad is a major triad; if a minor third and a perfect fifth, it is a minor triad. These are defined as consonant triads. If the third is major and the fifth is augmented, the triad is called an augmented triad; if the third is minor and the fifth is diminished, the triad is a diminished triad. Augmented and diminished triads are dissonant.

 In actual music, any factor of a triad may be duplicated and reduplicated in any octave; this is called doubling and is found everywhere in polyphonic (polyphony) music, both vocal and instrumental. As long as the notes are exclusively triadic factors and their octave equivalents, the chord is still considered a triad. For further discussion of triads and their variants, see harmony: Classical Western harmony (harmony) and chord.

Mark DeVoto

 

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