The task of adapting a composition for different musical ensembles is called arranging or orchestration, may be undertaken by the composer or separately by an arranger based on the composer’s core composition.
A composition may have multiple arrangements based on such factors as intended audience type and breadth, musical genre or stylistic treatment, recorded or live performance considerations, available musicians and instruments, commercial goals and economic constraints. Based on such factors, composers, orchestrators and/or arrangers must decide upon the instrumentation of the original work.
In the 2010s, the contemporary composer can virtually write for almost any combination of instruments, ranging from a string section, wind and brass sections used in standard orchestras to electronic instruments such as synthesizers. Some common group settings include music for full orchestra (consisting of strings, woodwinds, brass and percussion), concert band (which consists of larger sections and greater diversity of woodwind, brass, and percussion instruments than are usually found in the orchestra), or a chamber group (a small number of instruments, but at least two).
The composer may also choose to write for only one instrument, in which case this is called a solo. Solos may be unaccompanied, as with works for solo piano or solo cello, or solos may be accompanied by another instrument or by an ensemble.
Composers are not limited to writing only for instruments, they may also decide to write for voice (including choral works, some symphonies (e.g., Beethoven’s ninth symphony, operas, and musicals). Composers can also write for percussion instruments or electronic instruments. Alternatively, as is the case with musique concrète, the composer can work with many sounds often not associated with the creation of music, such as typewriters, sirens, and so forth.
In Elizabeth Swados’ Listening Out Loud, she explains how a composer must know the full capabilities of each instrument and how they must complement each other, not compete. She gives an example of how in an earlier composition of hers, she had the tuba playing with the piccolo. This would clearly drown the piccolo out. Each instrument chosen to be in a piece must have a reason for being there that adds to what the composer is trying to convey within the work.
Arranging is composition which employs prior material so as to comment upon it such as in mash-ups and various contemporary classical works.] The process first requires analysis of existing music, and then rewriting (and often transcription) for an instrumentation other than that for which it was originally intended. It often (but not always) involves new supporting material injected by the arranger. Different versions of a composed piece of music is referred to as an arrangement.
Even when music is notated relatively precisely, as in Western classical music from the 1750s onwards, there are many decisions that a performer and/or conductor has to make, because notation does not specify all of the elements of musical performance. The process of deciding how to perform music that has been previously composed and notated is termed “interpretation”. Different performers’ or conductor’s interpretations of the same work of music can vary widely, in terms of the tempos that are chosen and the playing or singing style or phrasing of the melodies. Composers and songwriters who present their own music in a concert are interpreting their songs, just as much as those who perform the music of others. The standard body of choices and techniques present at a given time and a given place is referred to as performance practice, whereas interpretation is generally used to mean the individual choices of a performer.