Selective schools choose some or all of their students based on certain characteristics, most notably - aptitude/ability (e.g. grammar schools selecting pupils based on their performance in the 11+ test) or, in the case of some religious schools, faith. Selection was at the heart of the tripartite system introduced by the 1944 Education Act but became a less common feature of the UK education system with comprehensivisation. However, the growth of specialist schools and faith schools in the 1990s and 2000s, as well as academies and free schools and the current proposals to build new grammar schools, has seen the return of selection.