The Grand Staff

Now, watch what happens when you extend the range of the staves with ledger lines:

The C on the ledger line below the treble staff is the same C as the one on the ledger line above the bass staff.  It is usually referred to as "Middle C".

These two staves may be joined together to form a Grand Staff.

Here are the note names on the Grand Staff, with some ledger lines shown.  Click on the note names to hear the pitch.