Just as important as his goals was his contribution to Arsenal's tactics. Along with Chapman, Buchan contributed to Arsenal's development of the WM formation to fully exploit the relaxation of the offside law. Buchan's idea was to move the centre half from a roaming position in midfield to a "stopper" position in defence, with one forward brought back into midfield. This meant the offside trap was no longer the responsibility of the two full-backs, but the single central defender, while the full backs were pushed wider to cover the wings. Eventually the change in tactic would bring Arsenal great success in the 1930s.