Sonid: a new app to learn and practice music theory

Guitarist and app developer Martijn Michel founded Sonid, an app to learn and practice music theory. “Music theory is a very important part of music education, but perhaps the hardest to understand. Being a guitar teacher myself, I wanted to create an app for my students so they could practice and learn about music theory on their own.”
Sonid teaches you about music theory step by step. The app is Martijn’s solo project to provide all musicians with an easy and practical way to learn music theory. In the app you will learn about intervals, scales, ear training and chords and chord-progressions. You can also look up any subject in the wikipedia, or discuss subjects in the in-app forum.

It is freely available for Android and iOS. 
Check out the website: Learn music theory with Sonid app