Composers specify pitches, rhythm, and loudness, but by specifying what instruments play they also affect what musicians call timbre or tone color. Timbre is the quality of sound that distinguishes one instrument from another, even though they may be playing the same pitch and loudness.
The list of instruments in a piece is called its instrumentation. Of course one may imagine any combination of instruments, but certain combinations, or ensembles, are somewhat standardized. A rock band, for example, often consists of two electric guitars, an electric bass guitar, and a drum set. The most important ensemble used in film scores is the symphony orchestra, sometimes called just an orchestra (although not a "symphony," which refers instead to a form of music).
An orchestra consists of three large categories of instruments: string instruments, wind instruments, and percussion. The strings normally include between 20 and 30 violins divided into two sections, 10-15 violas (an instrument similar to but somewhat larger than the violin), 8-12 cellos, and 4-6 basses (also known as double basses or string basses, sometimes to distinguish them from electric bass guitars). An orchestra may also include one or two harps, but when a musician refers to "the strings," that normally means the bowed instruments of the violin family.
The wind instruments of the orchestra are further divided into woodwinds, which includes the flutes, single reeds, and double reeds, and the brass, which includes the buzzed-lip instruments. In a standard orchestra, there are four types of woodwinds each of which may be represented by two to four players: flutes, oboes, clarinets, and bassoons. The brass instruments include the horn (also known as the "French" horn, usually four players), the trumpet (usually 2 or 3 players), the trombone (usually 2 or 3 players), and the tuba (0 or 1 players).
Each of the sections of the orchestra may be augmented or varied depending on the piece. A piece may require no trumpets, for example, or may need to include a contrabassoon (a very low double reed). A flute player may be asked to switch to piccolo or a clarinet player to bass clarinet. This is especially true of the percussion section, which is not really standardized, although some instruments are more common than others. There are rarely more than about 4 percussionists and often none.
The art of combining timbres by assigning different melodies to different instruments is called orchestration, even when the music is not written for a symphony orchestra. Because film scores are recorded in a studio, it is possible to create unnatural combinations by controlling the placement of microphones and the relative levels of the instruments (the "mix"). Today, instruments are often recorded separately and then later mixed together, offering further flexibility. In order to maximize efficiency and speed, film composers often leave the details of orchestration to another person, called the orchestrator. However, the role of the orchestrator can vary significantly, from a person who does actually orchestrate and makes all the decisions regarding which instruments play when (a common practice in the 1930s through the 1950s), to someone who mainly works out the details of notation. In contemporary scores, composers usually make all the important orchestration decisions.
A composer or orchestrator writes down all the notated parts for the individual instruments together in a score. The conductor then uses this large book to see what each musician is playing. The following is one page from the score to Lawrence of Arabia:
