A clef identifies and originally was used with a single line, and identifies others only by their relationship to this.
This was what has since become the fourth line, the clef line, of the bass stave.
They could only be written (as they are yet) in one clef—namely, the F clef.
After a change of clef in the middle of a score this is, of course, not necessary.
She was so tall the girls always considered her in that clef.
In olden days any clef line might be taken with any number of lines above and below.
The papers insist it's a livre--clef; and I am certain the thing is selling on that account!
clef, klef, n. a musical character placed on the staff by which the absolute pitch of the notes is fixed.
Le Tarot Divinatoire: clef du tirage des cartes et des sorts.
He also observes that this oratorio is the first work in which the proper sharps and flats are generally placed at the clef.
1570s in a musical sense, "character on a staff to indicate its name and pitch," from Middle French clef (12c.) "key, musical clef, trigger," from a figurative or transferred use of classical Latin clavis, which had only the literally sense "key" (see slot (n.2)). In the Middle Ages, the Latin word was used in the Guidonian system for "the lowest note of a scale," which is its basis (see keynote). The most common is the treble, violin, or G-clef, which crosses on the second line of the staff, denoting that as the G above middle C on the piano.