notes between these, but the sound is relatively weak and not usually used in performance. The addition of an F attachment allows for intermediate notes to be played with more clarity. Pedal tones The pedal tone on B♭ is frequently seen in commercial scoring but much less often in symphonic music while notes below that are called for only rarely as they "become increasingly difficult to produce and insecure in quality" with A♭ or G being the bottom limit for most tenor trombonists. Some contemporary orchestral writing, movie or video game scoring, trombone ensemble and solo works will call for notes as low as a pedal C, B, or even double pedal B♭ on the bass trombone. Glissando The trombone is one of the few wind instruments that can produce a true glissando, by moving the slide without interrupting the airflow or sound production. Every pitch in a glissando must have the same harmonic number, and a tritone is the largest interval that can be performed as a glissando. 'Harmonic', 'inverted', 'broken' or 'false' glissandos are those that cross one or more harmonic series, requiring a simulated or faked glissando effect. Trills Trills, though generally simple with valves, are difficult on the slide trombone. Trills tend to be easiest and most effective higher in the harmonic series because the distance between notes is much smaller and slide movement is minimal. For example, a trill on B3/C4 is virtually impossible as the slide must move two positions (either 1st-to-3rd or 5th-to-3rd), however at an octave higher (B4/C5) the notes can both be achieved in 1st position as a lip trill. Thus, the most convincing trills tend to be above the first octave and a half of the tenor's range. Trills are most commonly found in early Baroque and Classical music for the trombone as a means of ornamentation, however, some more modern pieces will call for trills as well. Notation Unlike most other brass instruments in an orchestral setting, the trombone is not usually considered a transposing instrument. Prior to the invention of valve systems, most brass instruments were limited to playing one overtone series at a time; altering the pitch of the instrument required manually replacing a section of tubing (called a "crook") or picking up an instrument of different length. Their parts were transposed according to which crook or length-of-instrument they used at any given time, so that a