Extending the slide from one position to the next lowers the pitch by one semitone. Thus, each note in the harmonic series can be lowered by an interval of up to a tritone. The lowest note of the standard instrument is therefore an E – a tritone below B. Most experienced trombonists can play lower "falset" notes and much lower pedal notes (first partials or fundamentals, which have a peculiar metallic rumbling sound). Slide positions are subject to adjustment, compensating for imperfections in the tuning of different harmonics. The fifth partial is rather flat on most trombones and usually requires a minute shortening of the slide position to compensate; other small adjustments are also normally required throughout the range. Trombonists make frequent use of alternate positions to minimize slide movement in rapid passages; for instance, B3 may be played in first or fifth position. Alternate positions are also needed to allow a player to produce a glissando to or from a higher note on the same partial.