Other theoretical writings explore the relationship of a piece of art to its environment, from which he developed his concept of "sites" and "non-sites". A "site" was a work located in a specific outdoor location, while a "non-site" was a work which could be displayed in any suitable space, such as an art gallery. "Spiral Jetty" is an example of a sited work, while Smithson's non-site pieces frequently consist of photographs of a particular location, often exhibited alongside some material (such as stones or soil) removed from that location.