The zone of immunity (or immunity zone) around a warship is an area from which both plunging fire and direct enemy fire is less effective. The concept was a factor in battleship design and in tactics during engagements. Warships traditionally have vertical, or near vertical, belt armour which protects against missiles (shells) travelling horizontally, and horizontal deck armour, which protects against plunging (downward) shell fire. Belt armour is generally thicker than deck armour.