The unit was developed in 1906 as a measure of the amount of time a student has studied a subject. For example, a total of 120 hours in one subject—meeting 4 or 5 times a week for 40 to 60 minutes, for 36 to 40 weeks each year—earns the student one “unit” of high school credit. Fourteen units were deemed to constitute the minimum amount of preparation that could be interpreted as “four years of academic or high school preparation.”