An art model poses for any visual artist as part of the creative process, providing a visual reference for the human figure in a work of art. However, more than being simply the subject of art, models are often thought of as muses, a source of inspiration without whom the art would not exist. The most common types of art works that use models are figure drawing, figure painting, sculpture and photography, but almost any medium may be used. Art models are often paid professionals with skill and experience but are rarely employed full-time, and artists may also rely on friends and family to pose. Paid art models are usually anonymous and unacknowledged subjects of the work. Models are most frequently employed for art classes or by informal groups of artists that gather to share the expense of a model. Models are also employed privately by professional artists. Although commercial motives dominate over aesthetics in illustration, its artwork commonly employs models. For example, Norman Rockwell used his friends and neighbors as models for both his commercial and fine art work. An individual who is having their own portrait painted or sculpted is usually called a "sitter" rather than a model, since they are paying to have the work done rather than being paid to pose. Beginning with the Renaissance, drawing the human figure from living models was considered the most effective way to develop the skill of draftsmanship. In the twentieth century, it became established that it is best to draw from life, rather than copying two dimensional images such as photographs. In addition, an artist has a connection to drawing another human being that cannot exist with any other subject. Models for life drawing classes are usually nude. In the classroom setting, where the purpose is to learn how to draw the human form in all the different shapes, ages and ethnicity, there are no real limitations on who the model can be. In some cases, the model may pose with various props, one or more other models, against real or artificial background, in natural or artificial light. The role of art models has changed through different eras as the meaning and importance of the human figure in art and society has changed. Nude modeling, nude art and nudity in general are at times subject to social disapproval, at least by some elements in society.