Exhibitions in commercial galleries are often entirely made up of items that are for sale, but may be supplemented by other items that are not. Typically, the visitor has to pay (extra on top of the basic museum entrance cost) to enter a museum exhibition, but not a commercial one in a gallery. Retrospectives look back over the work of a single artist; other common types are individual exhibitions or "solo shows", and group exhibitions or "group shows"). The Biennale is a large exhibition held every two years, often intending to gather together the best of international art; there are now many of these. A travelling exhibition is an exhibition seen at several venues, sometimes across the world.