Defense Position (守備形式, Shubi Keishiki, called Defense Mode in the anime) is a Battle position in which a Monster Card can be either face-up or face-down, in a Horizontal Position. In this position, the DEF value is used to represent this card, except where a card effect requires a specific examination of the ATK value.
In Defense Position, a Monster Card is always in a Horizontal Position. When a monster is Set, it is played in face-down Defense Position. Monsters cannot be Normal Summoned in face-up Defense Position, except by a card effect that allows them to be summoned that way (such as "Light of Intervention"). During a battle involving a Defense Position monster, the DEF value of that monster is used to calculate Battle Damage, rather than the ATK value.
Usually, Defense Position monsters cannot attack, but some monsters such as "Elemental HERO Rampart Blaster" and "Total Defense Shogun" have effects that allow them to attack while in Defense Position.
Also, the controller of a Defense Position monster takes no damage if it is destroyed in battle, even if the attacker has higher ATK than his monster's DEF. Exceptions do exist, as some cards grant the piercing ability, which lets a monster inflict damage in this way. Examples include monsters such as "Spear Dragon" and "Gaia Soul the Combustible Collective", as well as Spell and Trap Cards such as "Big Bang Shot" and "Dragon's Rage". If the attacker has lower ATK than the DEF of the attack target, the attacker takes Battle Damage, but neither monster is destroyed. If the attacker's ATK is equal to the target's DEF, neither player takes Battle Damage and neither monster is destroyed.
In the Yu-Gi-Oh! 5D's anime, Defense Position monsters become blue-colored.