So called 'zero-rating' refers to services that make certain content or applications available at no, or no additional, cost to the customer, and data volumes used to access the specified site or application do not contribute towards the customer's data usage. A user of a service provider offering Wikipedia Zero, for example, has unlimited, no-cost access to everything in the online encyclopaedia. Facebook's Free Basics provides clients of certain mobile-network providers free access to a limited number of websites and applications. While the three largest carriers in Kenya offered at least one zero-rated service, zero-rating was only proposed by one operator in Nigeria.