<P> The All England Club, through its subsidiary The All England Lawn Tennis Ground plc, issues debentures to tennis fans every five years to raise funds for capital expenditure . The original debentures were issued in 1920 . Each debenture provides a pair of tickets for each day of the tournament for five years . Only debenture holders are legally permitted to sell on their tickets to third parties . </P> <P> The Club was the venue for the tennis event at the 2012 Summer Olympics . </P> <P> The Club currently has 18 tournament grass courts, eight American clay courts, two acrylic courts and five indoor courts . There are also 22 Aorangi Park grass courts, which serve as competitors' practice courts before and during The Championships . The grass courts can be used from May until September . The grass has been cut to 8 mm since 1995, and 100% perennial ryegrass has been used for its strength since 2001 (prior to that, it was 70% perennial rye and 30% creeping red fescue). The courts are renovated in September, using nine tons of grass seed annually . </P> <P> The largest court is Centre Court, which usually hosts the finals of the main singles and doubles events at The Championships . The quotation above the players' entrance to Centre Court is an extract from the poem "If --", by Rudyard Kipling, which reads: "If you can meet with triumph and disaster and treat those two impostors just the same ." This court also served as the main venue for the tennis events at the 2012 Summer Olympics . Initially, the courts were arranged in such a way that the principal court was situated in the middle with the others arranged around it; hence the title "Centre Court". </P>

How many tournament grass courts does wimbledon have