On any harmonica, it is possible to play in more than one key even though there is a specific key written on the harmonica. This is because there are a variety of different sounds used in music and the major scale is only one of these sounds. The most common way of playing in another key is to use what is called second position or cross harp. This method is essential for Blues playing and is also used for other styles such as Country and Rock. When you play cross harp on the C harmonica, C is no longer the key note. The note G now becomes the key note. The note G can be found at holes 2, 3, 6 and 9 (see lesson 15 for detailed description). This method of playing can take some time to get used to, but is essential if you wish to pursue techniques such as note bending. The train imitation sounds you have already played have been cross harp in the key of G, so you’re halfway there already! If you are playing a 12 bar Blues in the key of G, the guitar would be playing the chords G (1), C (4) and D (5). On here there is a chart showing which key harmonica to choose for cross harp playing with every key used in music. The following 12 bar Blues in the key of G uses the cross harp position to playa riff based around the notes G, C and D which are the root (foundation) notes of the chords played by the guitar to accompany the harmonica.
Here is a 12 bar Blues in the key of G which contains whole notes along with quarter notes and quarter rests. It makes use of notes on holes 1, 2, 3 and 4.