Many composers are, or were, also skilled performers of music.
Since musical notation only incompletely expresses a piece of music, there is a whole continuum of possibilities concerning how much the performer determines the final form of the rendered work in a concert. Even in a classical piece of instrumental music in which all of the melodies, chords, and basslines are written out in musical notation, the performer has a good degree of latitude to add artistic interpretation to the work by varying her articulation and phrasing, choosing how long to make fermatas (held notes) or pauses, and — in the case of bowed string instruments, woodwinds or brass instruments — deciding whether to use expressive effects such as vibrato or portamento. For a singer or instrumental performer, the process of deciding how to perform music that has been previously composed and notated is termed "interpretation". Different performers' 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 are interpreting, 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.
Although a musical composition often has a single author, this is not always the case. A work of music can have multiple composers, which often occurs in popular music when a band collaborates to write a song, or in musical theatre, where the songs may be written by one person, the orchestration of the accompaniment parts and writing of the overture is done by an orchestrator, and the words may be written by a third person.
A piece of music can also be composed with words, images, or, in the 20th and 21st century, computer programs that explain or notate how the singer or musician should create musical sounds. Examples of this range from wind chimes jingling in a breeze, to avant-garde music from the 20th century that uses graphic notation, to text compositions such as Aus den sieben Tagen, to computer programs that select sounds for musical pieces. Music that makes heavy use of randomness and chance is called aleatoric music, and is associated with contemporary composers active in the 20th century, such as John Cage, Morton Feldman, and Witold Lutosławski.
Music composed in the Baroque era, particularly in slow tempos, often was written in bare outline, with the expectation that the performer would add improvised ornaments to the melody line during a performance. Such freedom diminished in later eras, reflected in the increased use by composers of detailed notation, including marks of expression telling the performer more precisely how the work should be executed. A culture eventually developed whereby faithfulness to the composer's written intention came to be highly valued (see, for example, Urtext edition). This musical culture is almost certainly related to the high esteem (bordering on veneration) in which the leading classical composers are often held by performers.