An F trombone is an outdated, rarely used type of trombone that was most common during the Romantic period. It is no longer manufactured for the mass music-playing market, although some trombone players own custom versions. Generally, the old style F trombone was a bass trombone. These instruments are essentially historical, obsolete instruments that are playable but rarely included in orchestra arrangements, and they are usually only used in performance if one of the performers happens to own one. Some contemporary trombones are equipped with attachments called F triggers, which effectively lengthen the body of the trombone, allowing it to temporary convert into an F trombone.
Modern bass trombones differ from the F trombone style because they have wider pipes, they are tuned to B flat, and they are equipped with triggers that tune the instrument to F or D to allow it to play lower notes. Bass F trombones can also be called European-style bass trombones, and in addition to F, they can also come in G. On a whole, bass trombones are larger and play lower pitches than the trombones most people are familiar with, which are generally B flat tenor trombones. A tenor trombone that turns into an F trombone using a trigger is called a B flat/F trombone.
When an instrument is said to have a pitch, like F, G, or B flat, it means that the instrument sounds at that note when the instrumentalist plays a written C. The reason this is done is to center the notes for the instrument on the most readable part of the staff, preventing the use of excessive ledger lines. When a composer wishes to use notes that are outside of the main staff, she can draw short, dash-like lines above or below the staff to act as the staff for higher or lower notes. Since an instrumentalist usually plays notes centered on the main staff, using many ledger lines can make the music more difficult to read.
Often, the given pitch of an instrument indicates the fundamental pitch of the instrument, which is the note that plays from an instrument when it is in its home position with no keys pressed. On a trombone, this is the note that plays when the outer slide is drawn all the way closed. In effect, an F trombone would sound at F when played with the slide closed, and an F trombone player might have transposed music that shows a notated C for a sounded F.