For pianists, organists, and other musicians: This app shows how to modulate from one key to another key with a few simple chords. Simply select the original key signature and the new key signature. You will see the chords to play in the score window. Major and minor keys are supported. At the bottom of the screen are the chord names and a brief description of the chord's relationship to the original key and the new key. Swipe sideways over the score to hear the chords being played. The transition chords may be shown in various inversions by swiping up or down over the chord.
The chords are generated based on the simple rules that were used by Carlos Salzedo and Lucile Lawrence in there book "The Art of Modulating" printed in 1950. Although this fine book is still in print, it is difficult to carry the book with you. No need to scan through hundreds of examples from the book when one app on your phone or tablet can generate you what you want instantly.