Identity (music)

For other uses, see Limit (music) § Identity.

048 equals itself when transposed by 4 or 8 or when inverted

Sum-4 family () and interval-4 family ()

Sum-4 family and interval-4 family in the chromatic circle, symmetry easily seen

Sum-3 family and interval-3 family for comparison

In post-tonal music theory, identity is similar to identity in universal algebra. An identity function is a permutation or transformation which transforms a pitch or pitch class set into itself. Generally this requires symmetry. For instance, inverting an augmented triad or C4 interval cycle, 048, produces itself. Performing a retrograde operation upon the tone row 01210 produces 01210. Doubling the length of a rhythm while doubling the tempo produces a rhythm of the same durations as the original.

In addition to being a property of a specific set, identity is, by extension, the "family" of sets or set forms which satisfy a possible identity. These families are defined by symmetry, which means that an object is invariant to any of various transformations; including reflection and rotation.

George Perle provides the following example:

"C-E, D-F♯, E♭-G, are different instances of the same interval [interval-4]...[an] other kind of identity...has to do with axes of symmetry [reflection symmetry rather than interval families' rotational symmetry]. C-E belongs to a family [sum-4] of symmetrically related dyads as follows:"

		D		C♯		C		B		A♯		A		G♯
		D		D♯		E		F		F♯		G		G♯
		2		1		0		e		9		8		7
+		2		3		4		5		6		7		8
		4		4		4		4		4		4		4

C=0, so in mod12, the interval-4 family:

		C		C♯		D		D♯		E		F		F♯		G		G♯		A		A♯		B
		G♯		A		A♯		B		C		C♯		D		D♯		E		F		F♯		G
		0		1		2		3		4		5		6		7		8		9		t		e
−		8		9		10		11		0		1		2		3		4		5		6		7
		4		4		4		4		4		4		4		4		4		4		4		4

Thus, in addition to being part of the sum-4 family, C-E is also a part of the interval-4 family (in contrast to sum families, interval families are based on difference).

See also

• Klumpenhouwer network
• Point reflection
• Derived row
• Twelve-tone technique#Invariance