If you are studying advanced music theory beyond diatonic harmony and moving into chromatic harmony, you know how challenging it is to remember every single new harmony.
Here is one-page summary of the "Neopolitan 6th" chord which is a major triad built off of flat-2, typically in the minor mode. N6 has the most amazing, distinctive poignant sound. A favorite of Beethoven and Chopin.
This handout details the art of voice leading for the N6, with musical models that illustrate the best-practices in a harmonic context.