This paper presents the development and evaluation of an extended reality (XR) application for music theory education. A strong foundation in musical patterns and concepts is essential for successful piano playing, as it provides the necessary tools for a pianist to interpret a piece of music and to communicate his or her own musical ideas, leading to the development of musicianship and confidence. The interactive XR application focuses on fundamental music theory concepts, including music notes, intervals, scales, and chords. A user study was conducted with a total of 15 participants, including 3 music teachers, 6 intermediate piano players, and 6 beginner piano players, to evaluate the effectiveness of the XR application. The results from the user study demonstrated a significant improvement in the understanding of music theory concepts. Participants, who were beginner and intermediate piano players, also provided positive feedback, indicating that the XR application provided an engaging and effective learning experience. These findings suggest that XR applications have the potential to support music theory education for beginner and intermediate piano players.