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A chromatic harmonica is a type of harmonica which is capable of playing all 12 notes notes of the Western chromatic scale. Unlike diatonic harmonicas it makes use of a slide activated button to redirect air from the mouth piece to the desired reed.
A simplified way to explain this would be to compare the chromatic harmonica to a piano. The slide button essentially allows you to access all of the sharps & flats in the desired scale much like the black keys on a piano. This makes the instrument incredibly versatile and allows the player to access any key.
I have been playing and performing chromatic harmonica for many years. I have always relished the challenge of playing the instrument in all 12 keys and have many students worldwide that seek me out for online lessons.