Social stratification is a system that separates human beings into groups, with one group enjoying more social privileges and status than another. Stratification is usually based on characteristics like race, class, ethnicity, wealth, caste, or education. Many stratification systems exist in a pyramid structure with fewer people at the top levels than the bottom levels. Stratification systems can be criticized as unjust, and they generally fall into open or closed categories depending on whether social mobility between strata is possible.