This is a simple app to transpose guitar keys for rhythm pattern to common suitable chords of that key. For example, in a typical G major key song the possible chord combinations are typically G, Em, C, D, Am, Bm and B (not considering diminished chords, but Bm and B both goes with G & Em keys, that is a different music theory but has very common usage). These chords can be in any combination and there might be other chords, but most of the songs would contain a variation of only these chords (more typically you will only see G, Em, C, D for a G major key song).

This app helps to transpose these frequently used chords if you are changing the key of the song. i.e. if you want to transpose the G major song to F major and you don't want to calculate the possible combinations (F, Dm, A#, C, Gm, Am, A).

The icon is taken from here

https://www.iconfinder.com/icons/49667/guitar_guitar_hero_music_rock_slash_icon#size=512

Which is Free for commercial use.