Developed in 1906, the Carnegie Unit measures the time a student has studied a subject in the U.S. secondary and postsecondary education system. It was originally conceived to translate high school work into equivalencies for the purpose of college admission: Students earn one unit of high school credit upon completing 120 hours in one subject, accumulated in four or five meetings a week for 40 to 60 minutes for 36 to 40 weeks each year. Fourteen units constitute minimum high school preparation.

The early decades of the 20th century were a period of massive expansion of high school populations, creating a good deal of articulation about the mission of the high school and its curriculum and increasing numbers of applicants for postsecondary education. National standards for ...