A brief explanation of "Crossharping"

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Diatonic harmonicas come in 12 different keys, one for each 1/2 step in a standard chromatic octave. If you play the harmonica in the key that it is labeled, you are playing straight harp or first position. You would be using mostly blow notes, some draw note, and few bends. This is the position in which most "folk" style music is played and is well suited for playing melody lines. To play cross harp (second position), you choose a harmonica in a key other than the key in which the song is written (using the chart below). The main reason for doing this is that the draw notes can be bent for more expression and to get your "blue" notes (flatted third, fifth, and seventh) which are not naturally present on the harmonica in the key that directly corresponds to the key of the song. Cross harp (second position) is the position that is mostly used for blues, rock, and country. In these styles the harmonica is usually not playing the melody, but is used to add fills and riffs that compliment and add to the music.

So, for example, depending on the style of music being played, if the song is in the key of G, you could play a G harp in first position, or a C harp in second position and both would "sound right" with the music. The C harp could be made to sound more "bluesy" than the G, and would work better on blues, etc.