bass clef
Also known as the F clef.
The musical symbol (𝄢) used to designate notes on the bass staff, or in layman's terms, the lower range of notes. This most common of clefs is associated with bass voices, contrabass (a.k.a. double bass), bass guitar and other low-pitched instruments.
Kagamine Len has a bass clef on his shirt, whereas Kagamine Rin has a treble clef.