The organizing of teaching and learning has been structured since the first educational institutions were established. With greater specialization and expertise in a particular field of knowledge, and a gathering of like-minded individuals, instructors clustered into specialized groups, schools, and eventually departments within larger institutions. This structure spread rapidly during the 19th and 20th centuries with factory model schools and their "assembly-line" method of standardized curriculum and instructional methods.[1] Beginning with the progressive educational movement in the early-mid 20th century, and again with similar trends in the late 20th and early 21st century, alternative models structured towards deeper learning, higher retention, and 21st century skills developed.[2] The organizational models of schools fall into several main categories, including: departmental, integrative, project-based, academy, small learning communities, and school-within-a-school.