A pit orchestra is a type of orchestra that accompanies performers in musicals, operas, and other shows involving music. The terms was also used for orchestras accompanying silent movies when more than a piano was used.  In performances of operas and ballets, the pit orchestra is typically similar in size to a symphony orchestra, though it may contain smaller string and brass sections, depending upon the piece. Such orchestras may vary in size from approximately 30 musicians (early Baroque and Classical Opera) to as many as 90-100 musicians (Wagnerian Opera). However, because of financial, space, and volume concerns, the musical theatre pit orchestra is considerably smaller (at most 20-30 musicians, including not more than ten string players).
Typically, pit orchestras play in a lowered area in front of the stage called an orchestra pit. Inside the pit, the conductor stands facing towards the stage with his or her back towards the audience to coordinate the music with the actions of the singers, dancers and actors; while the orchestra sits facing the conductor. If a performance is funded enough, there may also be a camera that broadcasts what the conductor is doing live to TV screens at the back of the theatre. The conductor may also sit at a keyboard (or two) and conduct as well as play. This is often the case when a show only requires a small orchestra, or on national tours, where the instrumentation is often reduced from the original arrangement and one or two keyboard players substitute for several instruments.
Music parts for pit orchestra woodwind players in musical theatre are normally divided into "Reed Books". Orchestration varies with each show based on the type of music that will be performed, such as jazz, classical, or blues. For example, a Reed 1 Book may contain music for Piccolo, Flute, Eb Alto Saxophone, Bb Clarinet, and/or Oboe. A musician handed a Reed Book would be expected to play each part. Because the musician plays so many different instruments, he or she is referred to as a "doubler" (even though the Reed Books may have up to eight instruments each).
Musicians who play in pit orchestras are not only required to play multiple instruments at times, but they must also be familiar and able to play in multiple keys, styles, and tempos and make a switch instantaneously. The orchestration for a musical is written in a key best suited to the singer. Some keys are more difficult to play in than others because of the increased attention that greater amounts of sharps and flats require. Musicals also tend to have a number of styles which can range from a soulful ballad to an involved funk tune to a hard rock song. Many musicians have been trained to play in a certain style but in order to play in pit orchestras, one must be able to play them all. Because musicals are live, many elements can change from show to show; pit orchestra musicians consequently should be able to play different tempos every night and even skip through their music to a spot if an actor messes up.
Preparing To Play In A Pit Orchestra
Although members of a pit orchestra are not required to demonstrate great stage presence, and they may work out of sight from much of the audience, they can generally be seen from the balcony seats and are thus required to adhere to standard rules of dress and appearance 
Preparation by musicians in a pit orchestra may consist of buying a recording of a show, listening to it, and play along with the recording.
Pit Orchestra Size Examples
Pit orchestras can range from large orchestras to small rock combos. While a pit orchestra usually plays in the orchestra pit, there are times when they are on stage in the background (this is usually for rock musicals). Below are examples pit orchestra examples from five major theatrical license companies: Music Theatre International, Tams-Witmark, Samuel French, Inc., Rodgers and Hammerstein Theatricals, and Theatrical Rights Worldwide (excluding any conductor scores unless needed). You can easily see the varying sizes of pit orchestras. Note that string parts are often written with the intent of having 2 musicians play a specific part, especially in older musicals.
Note that this orchestration is slightly different than the orchestration of the original Broadway production, which also called for a third trombone, a fifth reed, a harp, and did not have a dedicated piano player.