The three clef signs used in modern notation are:
Moveable C Clefs
The alto clef is used for viola; the remaining moveable C clefs are not used, except the tenor clef is used extensively in cello, bassoon, trombone, (and euphonium) in études, solos, orchestral, and chamber music. Advanced tuba and string bass players need to know how to read it when using études and solos for other instruments. Trombone players also need to know how to read alto clef.
The center line on the alto clef is Middle C.
Ranges are designated in scientific notation as follows:
Or: C - c - c' (middle C) - c" - c''' - c'''' - c''''' - c'''''' - c'''''''
In teaching beginning piano, I like to refer to
middle C, "C above" and "C below."
Ledger lines are frequently used to designate notes
above or below the staff:
Octaves may also be indicated by the "ottava" sign,
either below the staff (go down an octave)
or above the staff (go up an octave):