In music, what clef places middle C on the third line?

The clef that places middle C on the third line of the staff is the alto clef.

The alto clef is a C-clef: the center of its symbol identifies the line that carries middle C, also called C4. When that center is positioned on the third line, the clef is known as alto clef or viola clef. It places the written notes in a comfortable range for instruments and voices that often sit between treble and bass registers.

The viola is the instrument most strongly associated with alto clef, although it also appears in music for viola da gamba, alto trombone, mandola, and some other instruments. Historically, it was useful for writing alto vocal parts while avoiding excessive ledger lines.

A common mix-up is the tenor clef. It is also a C-clef, but it places middle C on the fourth line rather than the third. Bass clef places F below middle C on the fourth line, while treble clef places G above middle C on the second line. C-clefs can theoretically appear on other lines, but alto and tenor are the modern standard names.