Economists have tried to explain how income is distributed by looking at both the size and functional distribution of income. The functional distribution divides national income according to the main sources of income - rents, profits/interest, and wages. It focuses on three social classes. However, by the late 19th century the lines between classes were blurring. The size distribution instead looks at inequality between individual income earners regardless of source, and can include those with multiple sources of income across different classes.