ABC Session Viewer was initially developed for my Android tablet and phone so that I could display the tunes in Paul Hardy's Session Tunebook and change the chords for different capo positions on my guitar.
More features were added as I thought of them and as people at the Monday evening Blueshoots session suggested.
The software was written in a multi-platform way and so exactly the same source code could be used to build apps for Android and iPad (except that the iPad version did not play the tunes).
The iPad version was rewritten using Apple's new Swift programming language and in a way that also allowed it to run on an iPhone. However because it did not initially have all of the features of the original version it was released as Abc Viewer II so that the original Abc Session Viewer would remain available.