The Circle of chords app is good for learning the chord relatives. As it can be used for piano because it displays chords and sound samples it also shows the place of the griff as well as Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian scales. You can customize the background image, the screen orientation, the tuning, the hand you play with, the instrument, the notation of the chords and the scrolling method ( with the volume keys ). This app is not complex from the point of view of the informatics involved, but the music theory is displayed well and the app is easy to use. You can also use this app to learn how the chords sound like. Because you use this app, you will no longer need an ear trainer app. This app is not a big app and works on old Android versions.