Voicing
means harmonising a melody (or lead) with one or more instruments or voices, either with a similar instrument from the same section or with a combination. Block voicing is where the inside or harmony parts always move in the same direction as the lead. This type of harmony works well for the typical jazz orchestra (four trumpets, five saxophones, four trombones and rhythm section) but can also be used in many styles of pop and rock, e.g. for horn sections or backing vocals. Voicing can be used either on the actual melody or for a chordal accompaniment (backing).
Four- and five-part voicings are more straightforward than two- and three-part. This is because all four chord tones are used. With fewer than four voices decisions need to be made about which notes to omit.
Take a melody and add the three other notes of the chord beneath without omitting any.

As the melody of the Bb chord on beat one of bar 2 is the root, it has to be Bb6 not a Bbma7 to avoid a semitone interval at the top of the chord. Semitone intervals are no problem in inside parts. Unless a 6th chord lasts for more than one beat it is unnecessary to include it as a chord symbol for the rhythm section. (An exception would be if a rhythm section instrument was voicing chords with horns instead of comping)
Extensions
These are either defined by the melody or added to inside parts for colour. The rhythm section parts should include any extensions or alterations that last longer than one beat.
Extensions are usually treated as (unprepared) suspensions, 13th replaces 5th, 11th replaces 3rd, 9th replaces root.
Allowable extensions and altered extensions:
| Chord type: | Maj 7 | Maj 6 | Min 7 or min7b5 | Dominant 7 |
| 9 | 9 | 9 | 9, 11, 13 | |
| #11 | #11 | 11 | b9, b10 (#9) | |
| 13 (rare) | #11 (b5) | |||
| b13 (+5) |
Use with 4 part block voicing
| Extension | Omit | |
| 13 | 5 | Unusual in inside parts |
| b13 | 5 | Often treated as augmented 5th |
| 11 | 3 | Use sparingly in inside parts |
| #11 | 5 | Often treated as b5th |
| 13 or b13 with #11 | 5 and root | With only four parts the 9th would also be omitted as the 3rd and 7th are necessary |
| 9 or b9 | Root | Use freely in inside parts |
| b10 (#9) | Root | Often a suspensionof b9. Faster passages can omit 3rd instead of root for smoother voice leading, but does not sound as dissonant. |
9ths
9ths and altered 9ths are treated as suspensions of the root and always replace it, so the next chord note down is a 7th. In addition to 9ths in the lead, they can be freely used in inside parts for added interest. As with 6ths, unaltered 9ths do not need to be included in the rhythm section chord symbols unless they are used for the entire duration of a chord.

Ex 3: 9ths and altered 9ths.
11ths
11ths on a dominant 7 usually omit the 3rd, so can be viewed as slash
chords, eg a C11 (Gm7/C) would be voiced as a Gm7. (You can ignore the C as it is covered by the bass). 11ths on a minor 7 do not need to omit the 3rd, but for close voicing it is best to treat them the same as above.
13ths
13ths can be treated as suspensions of the 5th so the next note down is the 3rd.

Ex 4: 11ths and 13ths
#11ths
Usually a #11th can be treated as a b5, so the next note down is the 3rd.

Ex 5: #11ths (b5ths)
Passing notes
Passing notes are notes that occur in a step between two chord notes. In some cases they can be harmonised as if they are substituted for a note of the given chord (eg. a suspension or upper extension), or they can be harmonised with other passing notes to create a new passing chord (very often a passing diminished).

Ex 6: Passing notes
In ex 6A the Eb is harmonised as if it is an 11th or a suspended 4th. The F# is harmonised with a diminished chord. There is no movement between the first two notes of voices two, three and four. At slow tempos this is not a problem, but at medium and fast tempos there may be articulation problems, especially if the lead instrument is playing legato. The passing diminished chord in ex 6B is preferable as two of the three harmony voices are now moving. The harmonic impact of the passing chords become less important at faster tempos, but the need to minimise repeated notes in inside parts where the lead is moving becomes greater and it is usually possible to create movement in all voices.

Ex 7: Revoicing to give more movement in inside parts at fast tempos
TIP: It is a good idea to harmonise the chord notes before harmonising the passing notes.
A passing diminished should not usually be used with a dominant chord, instead use a minor7.

Ex 8: Passing note on a dominant harmonised as an extension
In ex 8 the passing note at beat three is harmonised as a 9th, replacing the root of the previous beat resulting in static inside parts. Where a passing chord is required for dominant chords you can usually use the minor 7th chord whose root is a fifth higher, in this case a Cmin7:

Ex 9: Passing note harmonised with a minor 7.
Here the lower part is moving nicely but the second and third parts are still static. The Eb is necessary to the F7 chord, especially at slower tempos, so will need to stay but a Cm9 can be used to give more movement:

Ex 10: Passing note harmonised with minor 9. (NB no root as 9th acts as suspension)
The Cm9 at beat 3 allows two of the three inside parts to move.
Chromatic Neighbour notes and auxiliaries
These can be harmonised with a chord of the same type moving in parallel or diminished chords:

Ex 11: The first non-chord note is a neighbour note harmonised in parallel, the second is a lower auxiliary harmonised with a diminished. Both of these harmonisations where chosen to allow movement in the inside parts.
Diatonic neighbour notes and auxiliaries
These are usually treated as suspensions or extensions. The following example shows how lower auxiliaries can be reharmonised on a IIm7-V7-I

Ex 12 (A): The F in the Cm7 is harmonised with an F7, the G in the F7 is harmonised with a Cm7. In 12 (B) the Cm9 is used to give more movement to the third part.
A typical harmonisation of this passage could also have used sustained inside parts:

Ex 13: sustained inside parts beneath auxiliary notes.
Changing tones (enclosure)

Ex 14: The first note is harmonised as a suspension, the second note as a lower chromatic neighbour note with a chord of the same type moving in parallel.
For open voicing the simplest method is to drop the second voice down an octave. Entire passages can be either open or close, or can use a combination.

Ex 15: Close and open voicing
Here the voicing is open on the Bb ma7 chord. This works well as the melody is moving by a larger interval and a b5 on the last beat of the V7 gives some strong voice leading at the cadence.