A Chromatic Scale is a scale that is made of only half-steps (H):
H-H-H-H-H-H, etc.
The Chromatic Scale starting on D is: D, D-sharp, E, F, F-sharp, G, G-sharp, A, A-sharp, B, C, C-sharp, D. (If we choose to go down the scale we use flats instead: D, D-flat, C, B, B-flat, A, A-flat, G, G-flat, F, E, E-flat, D).
Learn about more Scales