Musictheory.net is a terrific reference for music students and teachers. The online lessons begin with an explanation of the music staff, clefs, and ledger lines so students get a solid foundation. By the time the lessons are finished, students will understand and be able to build Neapolitan Chords. Every step of the way, students get a verbal explanation, a pictorial demonstration, and, when appropriate, an auditory example. You might want to use parts of this site to demonstrate elements of music theory in class, or perhaps assign a section to students for review as a study guide. Once you open a lesson, note the little keyboard in the upper right corner. When you click on that, you open a playable keyboard in a separate window.
courtesy of Knovation