Etymology
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interval (n.)
early 14c., "time elapsed between two actions or events," from Old French intervalle "interval, interim" (14c.), earlier entreval (13c.) and directly from Late Latin intervallum "a space between, an interval of time, a distance," originally "space between palisades or ramparts" [OED], from inter "between" (see inter-) + vallum "rampart, palisade, wall," which is apparently a collective form of vallus "stake," from PIE *walso- "a post" (see wall (n.)).

Metaphoric sense of "gap in time" also was in Latin. From c. 1400 in English as "a pause, an interruption in a state or activity." Musical sense "difference in pitch between two tones" is from c. 1600. Related: Intervallic.
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musical (adj.)

early 15c., "pertaining to music;" mid-15c., "tuneful, harmonious;" late 15c., "adept at making music," from Medieval Latin musicalis, from Latin musica (see music).  Related: Musically. Musical box is from 1829. Children's or parlor game musical chairs is attested from 1862, hence use of musical as a modifier meaning "changing rapidly from one to another possessor" (1924).

Instrumental and vocal music, the quadrille and country-dance, occupy a portion of the time. No waltzing is however permitted. After dancing, round games follow, as Terza, "The Post," Musical Chairs, Cross Questions, all tending to amuse and promote exercise, until the partial extinguishing of the gas, at ten p.m., gives warning of approaching bedtime. [The Rev. R. Wodrow Thomson, "Ben Rhydding, the Asclepia of England," 1862]

In mid-19c. makers of musical boxes also advertised musical chairs, "playing beautiful tunes simply by the weight of the person sitting in them."

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musical (n.)

"film or theatrical piece (other than opera) in which music figures prominently," 1937, from musical (adj.) in musical play. Earlier as a noun it meant "musical instrument" (c. 1500), "musical performance" (1570s); "musical party" (1823, a sense now in musicale).

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semitone (n.)

late 15c.,semiton, "a musical interval approximating one-half of a whole tone," what we would call a minor second, the smallest interval in ordinary scales, from Old French semiton and directly from Medieval Latin semitonus; see semi- + tone (n.) in the musical sense. In art, in reference to tints, by 1782. Related: Semitonal (1863); semitonic (1728).

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augmented (adj.)

c. 1600, "increased," past-participle adjective from augment. The musical sense of "greater by a semitone than a perfect or major interval" (opposite of diminished) is attested by 1825.

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complement (n.)

late 14c., "means of completing; that which completes; what is needed to complete or fill up," from Old French compliement "accomplishment, fulfillment" (14c., Modern French complément), from Latin complementum "that which fills up or completes," from complere "fill up," from com-, here probably as an intensive prefix (see com-), + plere "to fill" (from PIE root *pele- (1) "to fill").

From c. 1600 as "full quality or number, full amount;" musical sense of "simple interval that completes an octave from another simple interval" is from 1873. In 16c. also having senses which were taken up c. 1650-1725 by compliment.

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intermission (n.)

early 15c., "fact of intermitting, temporary pause," from Latin intermissionem (nominative intermissio) "a breaking off, discontinuance, interruption," noun of action from past participle stem of intermittere "to leave off, leave an interval," from inter "between" (see inter-) + mittere "let go, send" (see mission). Meaning "lapse of time between events" is from 1560s; specifically of performances (originally plays, later movies, etc.) from 1854.

Intermission is used in U.S. for what we call an interval (in a musical or dramatic performance). Under the influence of LOVE OF THE LONG WORD, it is beginning to infiltrate here and should be repelled; our own word does very well. [H.W. Fowler, "Modern English Usage," 1926]
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ninth (adj., n.)

"next in order or rank after the eighth; being one of nine equal parts into which a whole is regarded as divided;" c. 1300, nynþe, an alteration or replacement (by influence of nine) of nigonðe, from Old English nigoða, nigende, for which compare seventh. Also see -th (1). As a noun, "ordinal numeral corresponding to nine," late 13c. As the name of a musical interval, 1590s. Related: Ninthly.

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intermissive (adj.)

"not continuous," 1580s, from Latin intermiss-, past-participle stem of intermittere "leave off, leave an interval" (see intermit).

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blues (n.1)

"music form featuring flatted thirds and sevenths," possibly c. 1895 (though officially 1912, in W.C. Handy's "Memphis Blues"). Blue note "minor interval where a major would be expected" is attested from 1919, and at first was suspected as a source of the term.

I am under the impression that these terms [blue note, blue chord] were contemporary with, if they did not precede and foreshadow, the period of our innumerable musical 'Blues.' What the uninitiated tried to define by that homely appellation was, perhaps, an indistinct association of the minor mode and dyspeptic intonation with poor digestion; in reality, it is the advent in popular music of something which the textbooks call ambiguous chords, altered notes, extraneous modulation, and deceptive cadence. [Carl Engel, "Jazz: A Musical Discussion," The Atlantic Monthly, August 1922]
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