Learn to recognize the main chord types by ear. Major, Minor, Diminished, Augmented, Sus2, Sus4, Maj7, Min7, Dominant7, Half-diminished, Minor-Major, maj6, min6, 9, maj9, 7b9 are included for all keys, played as chords, as ascending arpeggios and as descending arpeggios.
All notes are also tagged, so you can concentrate on the material that is most difficult for you by using Filtered Decks.
The subdecks are sorted according to difficulty, so setting "New card gather order" to "Deck is the recommended setting for most users.