This paper provides insight in the process of developing a comprehensive and concise summary of the most prominent aspects of proficiency in elementary algebra at the end of secondary grade. It will serve as a theoretical frame of reference for devising or validating instruments for diagnosing the mastery of elementary algebra at the transit from school to university. A first draft had been based on literature, which then was presented to experts for further evaluation. The model now comprises ten aspects of proficiency which are allocated into a table of two dimensions, one referring to elements of algebraic language, the other referring to the cognitive actions performed on them.