Alto Clef

The Alto Clef is sometimes known as the C Clef because the indent of the clef sits on the middle line of the staff which corresponds to middle C.
Alto Clef Middle C Sheet Music

Alto Clef Notes

The Alto Clef notes are easy to work out as long as you focus on the fact that the middle line corresponds to middle C.
All of the other lines/spaces simply move up and down in step from there:
Alto Clef Notes

Why does the alto clef exist?

Historically, the Alto Clef was originally used to reduce the need for ledger lines when writing out the music for alto singers in choirs.
This is because the pitch range of an alto occupies an awkward range of notes that sits between the Treble Clef and Bass Clef staves.

If composers used a Treble Clef then they would end up putting lots of ledger lines in below the staff:
Alto Clef To Treble Clef Notes

Conversely, if they used the Bass Clef they would end up with lots of ledger lines above the staff:
Alto Clef To Bass Clef Notes

Using the Alto Clef meant that the typical vocal range of the alto fitted the staff without the need for lots of ledger lines.
Ironically the Alto Clef is no longer used to write out music for Alto singers!

Which instruments use the alto clef?

The Alto Clef is also sometimes called the Viola Clef because the viola is the most common instrument that the clef is used for.
This is because the range of the viola is lower than a violin and so fits neatly into the range of the Alto Clef.
You will also see the Alto Clef used for alto trombone parts.
Alto Clef Instruments

Alto Clef Scales

The key signatures for music written in the Alto Clef have the same numbers of sharps and flats as key signatures written in other clefs.
However, you may find it confusing when you see key signatures written in the Alto Clef because the sharps/flats will be on different lines/spaces than those written in Treble and Bass Clef.
For example, here is the key signature of E major (4 sharps) written in the Alto Clef, Bass Clef and Treble Clef:
Alto Clef E major key signature
You can see how the key signature is the same, but looks different due to which of the different clefs is used.