treble clef
Also known as the G clef.
The musical symbol (𝄞) used to designate notes on the treble staff, or in layman's terms, the upper range of notes. This most common of clefs is associated with soprano and alto voices, violin, flute and many other high-pitched instruments. In art, the treble clef is sometimes used as a general symbol for music.
Kagamine Rin has a treble clef on her shirt, whereas Kagamine Len has a bass clef.