Here is an extract from "Concerto No. 4 in D major, Movement 1". It is called Exposition and is written by Wolfgang Amadeus Mozart. In the first bar, there are five D's, all on ledger lines. We know that the top line is an F# as we are in the treble clef and there is an F# in the key signature. You can then count up from there and we get to D. We know the next note in beat 2 of the second bar is an F# as it is two lines above the D. The note after that is an A.

In the third bar, you can see that the ledger lines go very high. It would be quite difficult to count all the lines, so in this instance you should read the shape of the passage to find out the notes.

As you can hear, this part is quite high. You can therfore tell as the notes go higher on the stave, so does the pitch. Ledger lines are useful as you are able to play pitches higher that notes in the stave. This is also true for the opposite, if you go down the stave. The pitch goes down as the notes go lower.