Pathfinder's Rise of the Rolemaster
This social strata is composed of those who through birth, native talent or determination have acquired a level of education, knowledge or technical skill mastery that is uncommon and thus of value; clerks, scribes, clergy and sages as well as small traders, shopkeepers, successful artists and master craftsmen are examples of occupations that could support a middle class family. Members of the middle class do have to work for their living, but their skills usually allow them to have the choice between working for themselves or being employed by another. While usually not able to spend as lavishly as the upper class, members of the middle class still seek to patronize the arts and will purchase luxuries when they can.
Elarkish societies tend to have very strong middle classes. The Mannish societies around the Dalmatian Sea are generally experiencing a rising of their middle class. The middle class of Yrchish society is usually small but holds most of the authority and respect that Mannish society usually reserves for the Upper Class. In general, to support a middle class in a society requires both urbanization and a reason for people to move around; the larger the local population and the more travelers, from near or far, the greater the opportunity for the middle class to grow.
As their status in the middle class is dependent upon their particular specialized skill, members of this social strata are raised from birth valuing, practicing and using the traits that will help them master those skills.