The commander must be able to concentrate his force at the adversary's point of main effort. Concentration not only implies massing of forces but also massing of firepower. It includes such elements as movement, flexibility, and communications. At the lowest levels, concentration includes siting weapons and creating fire plans to mass fire effects on the attacker. Concentration cannot be achieved by being strong everywhere. Trading ground for time, or economy of force elsewhere, may be necessary to obtain an advantage at a decisive point. The defender uses deception, concealment, counter-battery fire, screening forces, and AD in order to minimize the risks of vulnerability through concentration of force.