Music theory is the study of the practices and possibilities of music. The Oxford Companion to Music describes three interrelated uses of the term "music theory": The first is what is otherwise called 'rudiments', currently taught as the elements of notation, of key signatures, of time signatures, of rhythmic notation, and so on.
Helpful Books is the developer of the app which is listed among the Books & Reference category. Music theory has been downloaded and set up by over 100K users and its most recent update is dated Sep 17, 2018. Users rated the tool 4 on Google Play.