Some of the PBS programs that OPB broadcasts come with requirements that they be aired on a certain day and within a certain time frame. These shows are often anchor programs of the PBS schedule - Masterpiece Theatre, NOVA, FRONTLINE, Washington Week, etc. Other programs may have no fixed schedule requirements, but do have a limited rights window during which OPB can broadcast the shows. OPB programmers take into account these and other factors as they build the TV schedule, grouping programs together in ways that serve as many segments of our audience as possible. Though most PBS stations use similar criteria, each one responds in different ways to the interests of its community, and so no two stations come up with identical schedules.