There are 10 criteria for a site to be able to be added to the list. World Heritage Site selection criteria i–iv are culturally based. selection criteria vii–x are natural criteria. Twenty-three properties are listed as "cultural". Four are listed as "natural". One is listed as "mixed", which means that the site is notable for both natural and cultural criteria. This is similar to the proportions of all World Heritage Sites. Of the 890 sites on the World Heritage List, 77.4% are cultural, 19.8% are natural, and 2.8% are mixed. St Kilda is the only mixed World Heritage Site in the UK. At first, it was added to the list because of its natural habitats. 