The credit hour originated around the turn of the 20th century, when the industrialist-turned-philanthropist Andrew Carnegie moved to create a pension system for college professors. Pensions were reserved for professors who worked full time, which ended up being defined as a minimum of 12 hours of classroom teaching per week in a standard 15-week semester. That’s why today, credit hours determine eligibility for student financial aid and graduation (you generally need 120 for a bachelor’s degree).