A musical-notation symbol at the beginning of a music staff, a clef indicates the pitch of the notes on the staff. Clefs were originally letters, identifying letter-named pitches, that were added to one or more of the staff's lines (thus providing a "key" to their identity). Knowing the identity of a single line permitted the musician to identify all the other lines and spaces above and below. Clefs were first regularly used in the 12th century. The Gothic letter forms of G and F evolved into the modern treble and bass clefs, respectively; the letter C evolved into the rarer alto, tenor, baritone, and soprano clefs.