The higher in the harmonic series any two successive notes are, the closer they tend to be (as evidenced by the progressively smaller intervals noted above). A byproduct of this is the relatively few motions needed to move between notes in the higher ranges of the trombone. In the lower range, significant movement of the slide is required between positions, which becomes more exaggerated on lower pitched trombones, but for higher notes the player need only use the first four positions of the slide since the partials are closer together, allowing higher notes in alternate positions. As an example, F4 (at the bottom of the treble clef) may be played in first, fourth or sixth position on a B trombone. The note E1 (or the lowest E on a standard 88-key piano keyboard) is the lowest attainable note on a B tenor trombone, requiring a full of tubing. On trombones without an F attachment, there is a gap between B1 (the fundamental in first position) and E2 (the first harmonic in seventh position). Skilled players can produce "falset" 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.