The cello, bassoon and trombone are the three main instruments that use the tenor clef. The tenor clef is mostly used by large, lower-pitched instruments whose range is both above and below middle C.
The tenor clef was created to reduce the amount of ledger lines and make the music easier for a musician to read.
Alto and tenor clefs are the same shape, since they are both classified as C clefs. However, the tenor clef is placed a little higher than the alto clef on a page of music. The only instrument that still uses the alto clef today is the viola.