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"Clef" is an anglicization of the French word for "key". In modern Western musical notation, a cleft is placed on the left side of a 5-line staff indicating the range of pitches (definition 1) available on that staff.
The use of clefs comes first from the concept of using a horizontal line drawn across the parchment to represent a specific pitch, to aid singers in learning the chants used in the Church.The line could represent any note, but was generally a low note (for the adult male singers), or a high note (for the boys), or a note somewhere between those two (for voices which fell in between).
The Dialogus of pseudo-Odo, written approximately the middle-900s, was the first music-theory treatise to name the pitches with the letters of the Roman alphabet in modulo-7 form, with A B C D E F G representing the diatonic pitches of the ascending scale, and repeating the letters in each octave (definition 1), thereby also employing octave-equivalence in practice for the first time in writing.
Eventually both of these concepts proved very useful, and scribes began to use a line for all three notes, with letters at the beginning to indicate which was which. The low note for the adult males was labeled "F", the high note for boys labeled "G", and the in-between note labeled "C". This is the origin of our modern clefs: the letter F evolved into the bass clef and the letter G evolved into the treble clef. C became the C-clefs which used to be used on every one of the 5 lines, and the two still in use are still moveable today (alto and tenor clefs), but its evolution from the letter C is not as obvious as the bass and treble clefs.
Additional clefs are also sometimes employed by composers, particularly for non-pitched percussion parts such as a drum-set notated on a 5-line staff, where the clef simply indicates that the notes are not to represent specific pitches but rather different parts of the drum-set.
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