The two main systems of social stratification are slavery and class system. In the slavery stratification system, ownership of people was advocated for after the rise of agricultural societies. In this prisoners of war were held captive and made to work on the farms. This system was more rampant in Greece and Rome, where prisoners of wars and debtors were traded amongst the society members. This later began to gain popularity in the European hemispheres as an exploration into other countries became more popular. A good example of this system in contemporary society is slavery in the US that entailed ownership of black people by the whites. The class system of stratification involves a person being born into a given social ranking but can move up and down the ranking easily. The up and down movement is determined by the efforts, knowledge and skills acquainted by the person. A good example of this is a person born into a middle social class who can move up the classes through the acquisition of knowledge and skills but can also move down to the low social class if they do not put efforts.