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A leger line or ledger line is a tool of musical notation to express notes that do not fall on the regular lines or spaces of the musical staff. A short line (slightly longer than the note) is drawn parallel to the lines on the staff (above or below as appropriate), corresponding to where the staff line would be if the note were in range (see Figure 1).
Notes on more than three or four leger lines above or below the staff are usually considered too hard to read, and if there are several measures of them, it is usually preferable to switch clef or use 8va notation. Some transposing instruments (such as the piccolo, the double bass), the guitar, and the tenor voice transpose at the octave in order to avoid leger lines.
Players of certain instruments, however, prefer leger lines to clef changes or 8va notation. Clarinetists, for example, would rather read leger lines in the chalumeau register than read bass clef notes, and flute players would rather read leger lines for notes in the third octave than read 8va notation because higher flute notes require different fingerings.