<Li> Category 4 (medium - fast) </Li> <Li> Category 5 (fast) </Li> <P> Of the current four Grand Slam tournaments, the Australian and US Open use hard courts, French Open is played on clay, and Wimbledon, the only Grand Slam to have always been played on the same surface, is played on grass . The Australian Open switched to hard courts in 1988 and in its early years the French championship alternated between clay and sand / rubble courts . The US Open is the only major to have been played on three surfaces; it was played on grass from its inception until 1974, clay from 1975 until 1977 and hard courts since it moved from the West Side Tennis Club to the National Tennis Center in 1978 . </P> <P> ITF uses the following classification for tennis court surface types: </P>

What are the tennis courts at wimbledon made from