From 2003/04 to 2008/09, we lived firmly in the Big Four era. Arsenal, Chelsea, Manchester United and Liverpool had a near-monopoly on the four Champions League places, with just one exception: Everton beat Liverpool into fourth place in 2004/05.
Manchester City and Tottenham joined the party to make it a Big Six from 2009/10 onwards. In the 13 seasons between then and 2021/22, those six teams made up the top six on six occasions.
