In music composition and arranging, a voicing is the instrumentation and vertical spacing and ordering of the pitches in a chord: which notes are on the top or in the middle, which ones are doubled, which octave each is in, and which instruments or voices perform each.
For example, the following three chords are root-position C major triads voiced differently:
All three voicings above are in root position, while the first is in close position, the most compact voicing, and the second and third are in open position, which includes wider spacing. In triadic chords, close root position voicing is the most compact voicing in thirds which has the root in the bass. Open and closed harmony are harmony and harmonization constructed from open and close position chords, respectively.
Many composers, as they developed and gained experience, became more enterprising and imaginative in their handling of chord voicing. For example, the theme from the andante movement of Beethoven’s early Piano Sonata No. 10 (1798), presents chords mostly in closed position:
Whereas in the theme of the Arietta movement that concludes his last Piano Sonata No. 32, Op.111 (1822), composed some twenty years later, Beethoven presents the chord voicing in a much more daring way, with wide gaps between notes, creating compelling sonorities that enhance the meditative character of the music:
Philip Barford (1971, p.147) describes The Arietta of Op. 111 as "simplicity itself … its widely-spaced harmonization creates a mood of almost mystical intensity. In this exquisite harmonization the notes do not make their own track – the way we play them depends upon the way we catch the inner vibration of the thought between the notes, and this will condition every nuance of shading." Ravel’s "Pavane de la Belle au Bois Dormant" (Pavane for the Sleeping Beauty) from his 1908 suite Ma Mère l’Oye (Mother Goose) exploits the delicate transparency of voicing afforded through the medium of the piano duet. Four hands can cope better that two when it comes to playing widely-spaced chords. This is especially apparent in bars 5-8 of the following extract:
Speaking of this piece, Austin (1966, p. 172) writes about Ravel’s technique of “varying the sonority from phrase to phrase by telling changes of register.” 
The two chords that open and close Stravinsky's Symphony of Psalms have distinctive sonorities arising out of the voicing of the notes. Austin (1966, p. 334) remarks: "The ﬁrst and last chords of the Symphony of Psalms are famous. The opening staccato blast, which recurs throughout the ﬁrst movement, detached from its surroundings by silence, seems to be a perverse spacing of the E minor triad, with the minor third doubled in four octaves while the root and ﬁfth appear only twice, at high and low extremes... When the tonic C major ﬁnally arrives, in the last movement, its root is doubled in ﬁve octaves, its ﬁfth is left to the natural overtones, and its decisive third appears just once, in the highest range. This spacing is as extraordinary as the spacing of the ﬁrst chord, but with the opposite effect of super-clarity and consonance, thus resolving and justifying the ﬁrst chord and all the horror of the miry clay."
Melodic doubling in parallel is the addition of a rhythmically similar or exact melodic line or lines at a fixed interval above or below the melody to create parallel movement while octave doubling (and doubling at other intervals, also called parallelism) of a voice or pitch is a number of other voices duplicating the same part at the same pitch or at different octaves. The doubling number of an octave is the number of individual voices assigned to each pitch within the chord. For example, in the three images in the introduction above only one pitch is doubled, the G in the rightmost image (above).
Parallelism destroys, creates, or maintains independence of lines; for example, in deference to the practices of his day always requiring and desiring a degree of independence in all lines, in Bach's "Gigue" from his English Suite no. 1 in A Major, BWV 806, m. 38 note that neither thirds (at the beginning) nor sixths (at the end) are maintained throughout the entire measure, nor any interval for more than four consecutive notes, but rather that the bass line is given its own part.
Consideration of doubling is important when following voice leading rules and guidelines, for example when resolving to an augmented sixth chord never double either notes of the augmented sixth, while in resolving an Italian sixth it is preferable to double the tonic (third of the chord).
Some pitch material may be described as autonomous doubling in which the part being doubled is not followed for more than a few measures often resulting in disjunct motion in the part that is doubling, for example, the trombone part in Mozart's Don Giovanni.
One nomenclature for describing certain classes of voicings is the "drop-n" terminology, such as "drop-2 voicings", "drop-2-and-4 voicings", etc. (sometimes spelled without hyphens). This systems views voicings as being built from the top down (probably from horn-section arranging where the melody is a given). The implicit, non-dropped, default voicing in this system is where all voices are in the same octave, with the individual voices being numbered from the top down. The highest voice is the first voice or voice 1. The second-highest voice is voice 2, etc. There is no provision in this nomenclature for having more than one voice on the same pitch.
A dropped voicing is where one or more voices are lowered by an octave relative to the default state. Dropping the first voice is undefined; a drop-1 voicing would still have all voices in the same octave and would result in a new first voice. There is no provision in this nomenclature for indicating the dropping of voices by two or more octaves or for having the same pitch represented in multiple octaves.
A drop-2 voicing is where the second voice is lowered by an octave. For example, a C triad has three "drop-2 voicings": reading down from the highest voice, C E G (the G has been dropped an octave), G C E, and E G C, which can be heard as the voicings supporting the first three melody notes of the Super Mario Bros. video game theme.
These are the drop-2-and-4 voicings for G7: (reading from the top down) G D F B (F and B have been dropped an octave), F B D G, D G B F, and B F G D. Various drop combinations are possible, given enough voices, such as drop-3, drop-2-and-3, drop-5, drop-2-and-5, drop-3-and-5, etc.
- Benward & Saker (2003). Music: In Theory and Practice, Vol. I, p.269. Seventh Edition. ISBN 978-0-07-294262-0.
- Benward & Saker (2003), p.274.
- Benward & Saker (2003), p.276.
- Benward & Saker (2009). Music: In Theory and Practice, Volume II, p.74. Eighth edition. ISBN 9780073101880.
- Benward & Saker (2009), p.74.
- Corozine, Vince (2002). Arranging Music for the Real World: Classical and Commercial Aspects. Pacific, MO: Mel Bay. p. 7. ISBN 0-7866-4961-5. OCLC 50470629.
- Barford P. (1971, p.147) ‘The Piano Music – II’ in Arnold, D. and Fortune, N. (eds) the Beethoven Companion. London, Faber.
- https://www.youtube.com/watch?v=nGUoEQMeEuE, an orchestral version can also be heard at https://www.youtube.com/watch?v=T5oVgqIbOqw
- Austin, W. (1966) Music in the 20th Century. London, Dent.
- https://www.youtube.com/watch?v=LUGyAtcEFy8, the closing chord can be heard at 21:57 of this recording
- Austin, W. (1966, p334) Music in the 20th Century. London, Dent.
- Benward & Saker (2003). Music: In Theory and Practice, p.133, Vol. I. Seventh Edition. ISBN 978-0-07-294262-0.
- Benward & Saker (2009). Music in Theory and Practice: Volume II, p.253. Eighth Edition. ISBN 978-0-07-310188-0.
- Benward & Saker (2009), p.106.
- Guion, David M. (1988). The Trombone: Its History and Music, 1697-1811, p.133. Musicology: A Book Series, Vol. VI. Gordon and Breach. ISBN 2-88124-211-1.