Wow! This App is a work of genius. Not only is it an invaluable resource for any guitar songwriter to help build chord progressions that will work together, but it also gives you all the information you need to solo along to almost any song using just the basic minor pentatonic scale. Even if you don't want to write songs or solo it's worth getting as a simple to follow guide to almost any bar chord that you would ever wish to play. You don't need to know about Ionians and Mixolydians - this does it all for you. A must have App for any guitarist. And for the price of a coffee it's definitely a no-brainer.
Tim - Travellin' Strings
http://www.travellinstrings.com