A G-clef placed on the second line of a stave. Treble clef is the only G-clef that is still in use, thus, the two terms very often are used interchangeably. The treble clef is the most common clef in use and is usually the first clef learnt by music students. The clef is used to notate the parts of instruments such as a violin, flute, oboe, clarinet, or saxophone. The music of a guitar is also presented on the treble clef, however, they sound an octave lower than written.