The Harmony app is a free circle of fifths app for Android phones and tablets. It first displays a menu with the circle of fifths with a selector. You can use the classical circle of fifths or the open key version. There also is a classical open key setting, but it only changes the G# into A b and the D# into E b. The main distinctive feature of this app is that it has a configurable setting you can use to make your own circle of fifths. The user interface is great and the app is easy to use. The version of the circle you can modify is the open key classical circle, but there is a problem here. You can write anything here, as there is no processing going on about that. Apart from that the app also has a power off button that quits the app even if it leaves it processing, it does not close it totally. The app is great if you have the time to configure you custom setting all by yourself, but even if you do not, you can make use of the standard pre-settings for the classical circle of fifths.