Descriptions of growth in educational achievement often rely on the notion that higher-level students can do whatever lower-level students can do, plus at least one more thing. This article presents a method of supporting such descriptions using the data of a subject-area achievement test. Multiple content domains with an expected order of difficulty were defined within the Grade 8 National Assessment of Educational Progress (NAEP) in mathematics. Teachers were able to reliably classify items into the domains by content. Using expected percentage correct scores on the domains, it was possible to describe each achievement level boundary (Basic, Proficient, and Advanced) on the NAEP scale by patterns of skill that include both mastery and non-mastery, and to show that higher achievement levels are associated with mastery of more skills. We conclude that general achievement tests like NAEP can be used to provide criterion-referenced descriptions of growth in achievement as a sequential mastery of skills.