Knowledge and skill hierarchies organize categories of knowledge and skills into prerequisite learning paths or complexity sequences. These hierarchies are developed from different perspectives of research, context, and purpose. Some are metahierarchies that support learning outcome analysis and alignment with learning strategies and assessments, generally in one or more domains: intellectual or cognitive, attitudes or affective, and motor or psychomotor. In particular disciplines and organizations, comprehensive hierarchies operate as detailed guides for teaching and learning in libraries, catalogs, and curriculum frameworks. Examples include psychomotor skills in dentistry, procedural and conceptual taxonomies in mathematics, analytical hierarchies in intelligent informatics, and skill trees for designing games. Such hierarchies usually evolve from job, task, information, cognitive, content, or procedural analysis and engage expert review for validation when precise ...