A traditional way to harmonize a melody in top-down, close-position voicings for 5 saxophones is the 1st alto plays the melody note, the 2nd alto plays the chord tone below the melody note, the 1st tenor plays the next chord tone (below that of the 2nd alto), the 2nd tenor plays the next chord tone, and finally the baritone doubles the 1st alto an octave lower. While there are 5 saxophones, the chord voicing is actually 4-part harmony.
Here is an example of this method of writing for saxophones using a melody note of D (9) on a C7 chord (from the top down): 1st alto = D, 2nd alto = Bb, 1st tenor = G, 2nd tenor = E, and baritone = D (octave below 1st alto). Even if tensions of a chord (natural or altered 9, 11, or 13) are used for modern harmony, the voicing often follows this traditional concept of harmonization. As a variation of the example used above on a C7: 1st alto = D, 2nd alto = Bb, 1st tenor = A (13), 2nd tenor = F# (#11) or E, and baritone = D.
Top-down, close-position 4-part harmony (as described above for saxophones) is also used for the brass section. Usually, the 4 trumpet parts are written first and then the 4 trombones double the trumpet parts an octave lower or with variations of this technique.
When saxophones are joined with the brass in concerted ensemble writing, the 1st alto usually doubles one of the trumpet parts and the remaining saxes double either lower trumpet or trombone parts. Thus, there is a considerable amount of unison and octave doubling in traditional big band arranging.
Open or spread voicings can go beyond the 4-part harmony used in traditional close-position writing. In spread vocings, the trombones and/or lower saxophones play basic chord tones (such as 1-3-7 or 1-7-3) and the trumpets and/or higher saxophones play remaining chord tones and, importantly, include tensions to give the voicing a richer sound. Here is an example of a spread voicing with an Ellington flavor on a C7 chord (from the bottom up): C, G, Bb, E, A, D#. Spread voicings can use 5, 6, or 7-part harmony instead of being limited to 4-part. However, unison and octave doubling still occurs when 7-part harmony is used in concerted saxophone & brass passages.
While the focus of this discussion is on traditional big band harmony, I would like to say a few words about contemporary writing for big bands. Characteristics include a greater use of individual lines (linear writing), contrary motion, expanded harmonic structures (clusters, intervalic-based structures, etc.), and orchestration that crosses horn sections. Even with the use of advanced harmonic concepts, unison and octave doubling is also found in contemporary big band music.
The simple reason why harmonic doubling is commonly used in big bands is there are more horn players in the band than the number of individual notes in the harmonic structures used by big band arrangers.
Given this perspective of harmonic doubling in big band music, I have an idea to offer to big bands: to try using my 6-horn ensemble scores with doubled players on a part. This is described in my Mid-Size Ensemble article. So far, big bands have been reluctant to try it. However, as I envision this approach (as another option to playing these scores with 6 horns), it can give conventional big bands a fresh sound.
Bands are encouraged to experiment with this idea. For example, the 12+ horns can be used in different combinations at places in the score as well as playing together. Of prime importance is my concept of having 6 lines with distinct tone colors instead of the conventional sound of saxophone, trumpet, and trombone sections. This can be done by combining saxophones & woodwind doubles with the brass using various mutes (as well as playing open) to create an expanded palette of tone colors. With this approach, each wind instrument is used as an individual tone color -- and blended with other colors -- instead of as part of a section.
Interested bands can write to me from the Contact page to discuss these ideas and to inquire about scores.