X noteheads, also referred to as “cross” or “crossed” noteheads have a number of functions in modern music notation.
In percussion writing, they are the go-to for non-pitched metallophone instruments such as cymbals or tamtam or gong. For drum set, particularly in jazz or rock charts, cymbals are typically the most active part, and X noteheads help these stand out from the other parts on a 5 line staff.
In vocal music, x noteheads are often used tor spoken text, or for unvoiced sounds / vocal effects. In both instrumental and vocal writing, they can be used to indicate notes of indefinite pitch. And in jazz charts, X noteheads can be used to indicate “ghost” notes in a melodic line.
(for drum set writing, there is an actual “ghost” notehead, which is a regular notehead in parenthesis.)
Cross (x) noteheads can also be used as a special effect to indicate hand / finger damping of instruments such as guitar, or tuned percussion such as vibraphone.
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