Key Signatures Explained

When we talk about the key of a piece of music, we are referring to the scale that is used to make up the melodies and chords. Below is the C major scale. A song in the key of C major is based on the notes of the C major scale. Therefore, a song in C major does not normally have sharps or flats.A song in the key of G major is based on the G major scale. Below is the G major scale. Notice how in the G major scale there is an F sharp note. Therefore, when you play a song in G major, you will normally play the note F sharp instead of note F.
This could mean that there could be lots of sharps written in the score. Below is a piece of music in G major. Notice all the sharps in front of the F note (circled in red). As you can tell, this can make a music score cluttered and thus difficult to read.
To get around this, the main sharps or flats in a piece are grouped and placed at the beginning of each staff, just after the clef sign. This is called the key signature. Below is the same piece of music in G major (shown above) but with the key signature added at the beginning.
Any sharp or flat present in the key signature, causes all notes with that letter name in the piece to be sharpened or flattened (not just the one on a particular line or space, as is the case with accidentals).

Two important points to remember about key signatures

1) In writing them, note the position and order of the flats or sharps. For instance, the D major key signature in the treble clef is always written as shown below:

Likewise, in the bass clef, it is always written as shown below:

2) The key signature is always placed after the clef and before the time signature.

The table below shows you all the key signatures on the treble clef.

Below is a table showing you all them on the bass clef.