All the then 17 first-class counties contested the first two competitions in 1959 and 1960; the next season when all 17 entered was 1977, though the number of teams in any one year was never lower than 14 (in 1971).
Gloucestershire and Somerset entered a combined team for two seasons, 1967 and 1968.
At present, all 18 current first-class counties take part in the Second XI Championship along with the MCC Young Cricketers team.
The team added to make the number up to twenty was Marylebone Cricket Club Universities.
This is a limited overs competition with the teams forming zones in the initial stage.