During the Battle of Britain, the defence of the UK's airspace was divided up within RAF Fighter Command into four Groups, each comprising several airfields and squadrons. The groups involved, 10, 11, 12 and 13, saw very different levels of activity during the battle. No. 11 Group, responsible for the defence of London and the south-east saw the heaviest fighting, and pilots were often rotated among the groups to allow them to rest and recuperate after several weeks of fierce contact with the enemy. Each group was commanded by an Air Vice-Marshal, who served under the head of Fighter Command during the battle, Air Chief Marshal Hugh Dowding.