Diatonic Harmony: If we look at The Major Scale in the key of C (white keys on a piano) and we start on the C key and move up a note at a time C, D, E, F, G, A, B, C or we can think of those notes as numbers C=1, D=2, E=3, F=4, G=5, A=6, B=7, C=8 once we think in numbers we can use this Diatonic Harmony chart. If you hold down key 3 then chances are that the next note you want to press is 6 and then from 6 chances are you would want to move to 2 and so on. This of course is not set in stone but if you want to play along to your favorite song and you know what key its in then this chart helps you sound like you know what your doing.