Grades 1-5: the Rudiments
ABRSM requires all candidates taking practical exams above Grade 5 to have at least a Grade 5 in Music Theory.
The ABRSM music theory syllabus up to Grade 5 covers all rudiments relating to Western common practice music. A good grasp on knowledge of music theory up to grade 5 of the ABRSM syllabus would involve proficiency in identifying the scales and key signatures of all 24 major and minor keys, simple and compound time signatures, the four most commonly-used clefs, basic harmonic progressions, scale degrees, simple transposition, and most terms and signs commonly encountered in notation. Having a strong foundation in all these areas of knowledge is not simply helpful, but absolutely essential in progressing into higher grades in terms of instrumental performance, for these are the basic building blocks of Western common practice music.
Grades 6-8: Advanced Analysis and Musicianship
A solid grasp on Grade 5 music theory would form the foundation and pave the way for acquiring good sight-reading ability, good listening ability, as well as more advanced analytical ability in music, all of which should be in the toolbox of a competent performer, to help the performer make intelligent and informed musical decisions. Armed with the toolbox of foundational knowledge covered in Grades 1-5, Grades 6-8 focuses on more advanced harmonic concepts and their application to the analysis of actual music. These grades also focus on putting together one’s knowledge of handling various musical elements acquired from the earlier grades, and applying that knowledge into composition of music.
A good ability in music analysis is essential to the serious performer in so many ways. For example, being able to analyse the structure of a piece might help one better plan how to phrase a particular passage, apply the appropriate dynamics and articulations, or choose a suitable tempo. These are all decisions that should not be made arbitrarily, but after careful study of the music itself and perhaps the context surrounding the genesis of the work. Therefore, in order to become a better performer, it is important to not only gain a good grounding in the rudiments of music theory, but to solidly retain these skills and knowledge for more advanced application in the long run, in order to develop into a complete musician.