The key of C major was discussed in lesson 5. Songs that use notes from the C major scale are said to be in the key of C major. Similarly, songs that use notes from the G major scale are said to be in the key of G major. Songs in the key of G will usually contain F# notes.
Instead of writing a sharp sign before every F note on the staff, it is easier to write just one sharp sign after each clef. This means that all the F notes on the staff are played as F#, even though there is no sharp sign written before them. This is called a key signature.
This is the key signature for the key of G major. It has one sharp sign after each clef.
The C major scale contains no sharps or flats, therefore the key signature for the key of C major contains no sharps or flats.
This song is a traditional Irish dance tune, written here in the key of G major. Notice the key signature reminding you to play all F notes as F#. The left hand accompaniment contains broken chords played staccato. Take care when changing from G to D7 and practice the left hand by itself if you need to.