Air units may "stage" (move between bases up to twice their combat range - during staging or SR they may fly across water up to their combat range) and still be used for combat. Air units may Counterair (attack enemy air at their bases - British air factors in Malta may decline combat but must be inverted if they do so), give Ground Support to attackers (including exploiting armour) or Defensive Air Support. For Ground Support or DAS air factors up to three times the face value of the ground forces may be added; these factors can be removed as part of an exchange, to allow the survival of ground units, or may be eliminated if that side loses the combat altogether. While the attacker may intercept DAS, a numerically superior defender cannot add extra factors to this air battle; a common house rule was to allow the defender to fly "escorts" to fight off interceptors.