“Theory is the abstract knowledge or reasoning of any art as opposed to the practice of it.”
Music theory is a subject which examines how music works and how it is built. It is about studying the notation and language of music to be able to understand the structure and the pattern behind any piece of music. Studying music theory enables us to find out how different composers of different genres and eras create their music.
Today the knowledge of theory is a precondition of musical understanding. Knowing the grammar of music helps musicians to improve sight reading, and memorisation. It enables student to enjoy improvisation, composition or harmonising simple tunes.
In music the theory includes form, harmony, composition, ear training, esthetics, pitch, rhythm, melody, chords, dynamic, tempo etc.