View source for Interval vector

[[File:Z-relation Z17 example.png|thumb|right|350px|Example of [[#Z-relation|Z-relation]] on two pitch sets analyzable as or derivable from set [[List of set classes|5-Z17]]<ref name="Schuijer">Schuijer, Michiel (2008). ''Analyzing Atonal Music: Pitch-Class Set Theory and Its Contexts''. University of Rochester. {{ISBN|978-1-58046-270-9}}.</ref>{{rp|99}} {{audio|Z-relation Z17 example.mid|Play}}, with intervals between pitch classes labeled for ease of comparison between the two sets and their common interval vector, 212320.]]
[[Image:Interval vector C major chord.png|thumb|Interval vector: C major chord, set [[List of set classes|3-11B]], {0,4,7}: 001110.]]
[[File:Deep scale property of the diatonic scale.png|thumb|Diatonic scale in the [[chromatic circle]] with each interval class a different color, each occurs a unique number of times]]
[[File:Deep scale property of the diatonic scale notation.png|300px|thumb|C major scale with interval classes labelled; vector: 254361]]
[[File:Deep scale property lack of the whole tone scale notation.png|250px|thumb|Whole tone scale on C with interval classes labelled; vector: 060603]]

In [[Set theory (music)|musical set theory]], an '''interval vector''' is an array of [[natural number]]s which summarize the [[Interval (music)|interval]]s present in a [[Set (music)|set]] of [[pitch class]]es.  (That is, a set of [[Pitch (music)|pitch]]es where [[octave]]s are disregarded.)  Other names include: '''ic vector''' (or interval-class vector), '''PIC vector''' (or pitch-class interval vector) and '''APIC vector''' (or absolute pitch-class interval vector, which Michiel Schuijer states is more proper.)<ref name="Schuijer"/>{{rp|48}}

While primarily an analytic tool, interval vectors can also be useful for composers, as they quickly show the sound qualities that are created by different collections of pitch class. That is, sets with high concentrations of conventionally dissonant intervals (i.e., seconds and sevenths) sound more dissonant, while sets with higher numbers of conventionally consonant intervals (i.e., thirds and sixths) sound more [[consonance and dissonance|consonant]]. While the actual perception of consonance and dissonance involves many contextual factors, such as [[Register (music)|register]], an interval vector can nevertheless be a helpful tool.

==Definition==

In [[equal temperament|twelve-tone equal temperament]], an interval vector has six digits, with each digit representing the number of times an [[interval class]] appears in the set. Because interval classes are used, the interval vector for a given set remains the same, regardless of the set's [[permutation (music)|permutation]] or vertical arrangement. The interval classes designated by each digit ascend from left to right. That is:
# minor seconds/major sevenths (1 or 11 semitones)
# major seconds/minor sevenths (2 or 10 semitones)
# minor thirds/major sixths (3 or 9 semitones)
# major thirds/minor sixths (4 or 8 semitones)
# perfect fourths/perfect fifths (5 or 7 semitones)
# tritones (6 semitones) (The tritone is [[Inversional equivalency|inversionally equivalent]] to itself.)

Interval class 0, representing unisons and octaves, is omitted.

In his 1960 book, ''The Harmonic Materials of Modern Music'', [[Howard Hanson]] introduced a [[monomial]] method of notation for this concept, which he termed ''intervallic content'': p''{{sup|e}}''m''{{sup|d}}''n''{{sup|c}}''.s''{{sup|b}}''d''{{sup|a}}''t''{{sup|f}}'' for what would now be written {{angbr|''abcdef''}}.<ref name="Hanson-1960"/><ref group="note">To quantify the consonant-dissonant content of a set, Hanson ordered the intervals according to their dissonance degree, with '''p'''='''p'''erfect fifth, '''m'''='''m'''ajor third, '''n'''=mi'''n'''or third, '''s'''=major '''s'''econd, '''d'''=(more '''d'''issonant) minor second, '''t'''='''t'''ritone.</ref> The modern notation, introduced by Donald Martino in 1961, has considerable advantages{{specify|date=September 2018}} and is extendable to any [[equal division of the octave]].<ref name="Martino-1961">{{Cite journal |last=Martino |first=Donald |date=1961 |title=The Source Set and Its Aggregate Formations |journal=Journal of Music Theory |publication-place=New Haven |publisher=Yale University Press |volume=5 |number=2 |page=224-273 |jstor=843226 |doi=10.2307/843226 }}</ref> Allen Forte in his 1973 work ''The Structure of Atonal Music'' notated the interval vector using square brackets, citing Martino;<ref name="Forte">{{Cite Q|first=Allen |last=Forte |author-link=Allen Forte |date=1973 |Q130092153}}</ref>{{rp|15}} subsequent authors, e.g. [[John Rahn]], use angled brackets.<ref name="Rahn-1980">[[John Rahn|Rahn, John]] (1980). ''Basic Atonal Theory''. New York: Longman. {{ISBN|9780582281172}}. Reprinted 1987, New York: Schirmer Books; London: Collier Macmillan. {{ISBN|0-02-873160-3}}.</ref>{{rp|100}}

A scale whose interval vector has six unique digits is said to have the [[deep scale property]]. The major scale and its modes have this property.

For a practical example, the interval vector for a C [[major triad]] ([[List of set classes|3-11B]]) in the root position, {C E G} ({{audio|Major triad on C.mid|Play}}), is {{angbr|001110}}.  This means that the set has one major third or minor sixth (i.e. from C to E, or E to C), one minor third or major sixth (i.e. from E to G, or G to E), and one perfect fifth or perfect fourth (i.e. from C to G, or G to C). As the interval vector does not change with transposition or inversion, it belongs to the entire [[set class]], meaning that {{angbr|001110}} is the vector of all major (and minor) triads. Some interval vectors correspond to more than one sets that cannot be transposed or inverted to produce the other. (These are called [[#Z-relation|Z-related sets]], explained below).

For a set of ''n'' pitch classes, the sum of all the numbers in the set's interval vector equals the [[binomial coefficient]] <math>\tbinom{n}{2} = \tfrac{n (n-1)}{2}</math>, since the interval vector elements are computed comparing each pair of pitch classes from the set consisting of n elements. This corresponds also to the [[triangular number]] 
<math>T_{n-1}</math>.

An expanded form of the interval vector is also used in [[transformation theory (music)|transformation theory]], as set out in [[David Lewin]]'s ''Generalized Musical Intervals and Transformations''.{{Full|date=August 2015}}

==Z-relation== <!--[[Z-relation]] redirects directly here.-->

[[File:Z-related hexachords from Wozzeck.png|thumb|right|350px|Successive Z-related hexachords from act 3 of ''[[Wozzeck]]''<ref name="Forte"/>{{rp|79}} {{audio|Z-related hexachords from Wozzeck.mid|Play}}]]

In musical set theory, a '''Z-relation''', also called '''isomeric relation''', is a relation between two pitch class sets in which the two sets have the same intervallic content (and thus the same interval vector) but they are not [[transposition (music)|transposition]]ally related (are of different T<sub>''n''</sub>-type ) or [[inversion (music)|inversion]]ally related (are of different T<sub>''n''</sub>/T<sub>''n''</sub>I-type).<ref name="Schuijer"/>{{rp|99}} For example, the two sets 4-z15A {0,1,4,6} and 4-z29A {0,1,3,7} have the same interval vector {{angbr|111111}} but one can not transpose and/or invert the one set onto the other.

In the case of [[hexachord]]s each may be referred to as a '''Z-hexachord'''. Any hexachord not of the "Z" type is its own [[complement (music)|complement]] while the complement of a Z-hexachord is its Z-correspondent, for example 6-Z3 and 6-Z36.<ref name="Forte"/>{{rp|79}} See: [[Schoenberg hexachord|6-Z44]], [[all-trichord hexachord|6-Z17]], [[Sacher hexachord|6-Z11]], and [[Forte number]].

The symbol "Z", standing for "[[zygotic]]" (from the Greek, meaning paired or [[yoke]]d, such as the fusion of two reproductive cells),<ref name="Schuijer"/>{{rp|98}} originated with Allen Forte in 1964, but the notion seems to have first been considered by Howard Hanson. Hanson called this the ''isomeric relationship'', and defined two such sets as ''isomeric''.<ref name="Hanson-1960">Hanson, Howard (1960). ''Harmonic Materials of Modern Music'' New York: Appleton-Century-Crofts. {{ISBN|0-89197-207-2}}.</ref>{{rp|22}} See: [[isomer]].

According to Michiel Schuijer (2008), the '''hexachord theorem''', that any two pitch-class complementary hexachords have the same interval vector, even if they are not equivalent under transposition and inversion, was first proposed by [[Milton Babbitt]], and, "the discovery of the relation," was, "reported," by [[David Lewin]] in 1960 as an example of the '''complement theorem''': that the difference between pitch-class intervals in two complementary pitch-class sets is equal to the difference between the cardinal number of the sets (given two hexachords, this difference is 0).<ref name="Schuijer"/>{{rp|96–7}}<ref name="Lewin">Lewin, David. "The Intervallic Content of a Collection of Notes, Intervallic Relations between a Collection of Notes and its Complement: an Application to Schoenberg’s Hexachordal Pieces", ''Journal of Music Theory'' 4/1 (1960): 98–101.</ref> Mathematical proofs of the hexachord theorem were published by Kassler (1961), Regener (1974), and Wilcox (1983).<ref name="Schuijer"/>{{rp|96–7}}

Though it is commonly observed that Z-related sets always occur in pairs, [[David Lewin]] noted that this is a result of twelve-tone [[equal temperament]] (12-ET).{{Citation needed|date=August 2015}}  In 16-ET, Z-related sets are found as triplets.  Lewin's student Jonathan Wild continued this work for other tuning systems, finding Z-related tuplets with up to 16 members in higher ET systems.{{Citation needed|date=August 2015}}

The equivalence relationship of `having the same interval content', allowing the trivial isometric case, was initially studied in crystallography and is known as [[Homometric_structures|Homometry]]. For instance the complement theorem is known to physicists as [[Babinet's principle]]. For a recent survey see.<ref name="Mandereau et alii">John Mandereau, Daniele Ghisi, Emmanuel Amiot, Moreno Andreatta, Carlos Agon. Z-relation and homometry in musical distributions. Journal of Mathematics and Music, Taylor & Francis (2011), 5 (2), 83-98.</ref>

Straus argues, "[sets] in the Z-relation will sound similar because they have the same interval content,"<ref>Straus, Joseph Nathan (1990). ''Introduction to Post-Tonal Theory'', p.67. 1st ed. Prentice Hall: Englewood Cliffs, New Jersey. {{ISBN|0-13-189890-6}}. Cited in Schuijer (2008), p.125.</ref><ref name="Schuijer"/>{{rp|125}} which has led certain composers to exploit the Z-relation in their work.  For instance, the play between {0,1,4,6} and {0,1,3,7} is clear in [[Elliott Carter]]'s [[String Quartet No. 2 (Carter)|Second String Quartet]].{{Citation needed|date=August 2015}}

===Multiplication===

Some [[#Z-relation|Z-related]] chords are connected by ''M'' or ''IM'' ([[multiplication (music)|multiplication]] by 5 or multiplication by 7), due to identical entries for 1 and 5 on the interval vector.<ref name="Schuijer"/>{{rp|83, 110}}

==See also==
*[[Interval cycle]]
*[[Pitch interval]]

==Notes==
<references group="note" />

==References==
<references/>

==External links==
*[http://www.mta.ca/faculty/arts-letters/music/pc-set_project/pc-set_new/pages/page06/page06.html Set classes and interval-class content]
*[https://www.robertkelleyphd.com/home/atnltrms.htm Introduction to Post-Functional Music Analysis: Post-Functional Theory Terminology, by Robert T. Kelley]
*[http://www.lsu.edu/faculty/jperry/virtual_textbook/20th_c_pitch_theory.htm Twentieth Century Pitch Theory: Some Useful Terms and Techniques]

{{Set theory (music)}}

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[[Category:Musical set theory]]