Face-down ( 裏 ( うら ) 側 ( がわ ) 表 ( ひょう ) 示 ( じ ) Uragawa Hyōji) is a position in which a card's backing is visible; as the term "face-down" suggests, the card's face is placed against the playing surface, preventing it from being seen. This is the opposite of face-up.
Face-down cards have no properties (name, Attribute, Type, Level/Rank, ATK, DEF, effect) until they are changed to face-up. A face-down card being looked at or revealed is still considered face-down (although its properties can be determined by card effects while being revealed) and is returned to its original position once the player(s) have finished viewing it.
In Yu-Gi-Oh! ARC-V, small orbs with glowing red eyes appear on top of face-down monsters.
Each player's Main and Extra Deck is placed face-down in their respective zone at the start of each game. Cards in the Main deck are always face-down, unless stated otherwise
Face-down cards in the Extra Deck can be looked at by their possessor at any time, but cannot be looked at by their opponent.
Extra Deck Pendulum Monsters (Fusion, Synchro, Xyz/Pendulum) can be summoned by its own mechanic only while face-down in the Extra Deck, and cannot be Pendulum Summoned.
During a game, cards are normally placed face-down by Setting them. Face-down cards on the field can be looked at by their controller at any time, but cannot be looked at by their opponent, unless stated otherwise.
Face-down cards on the field can be flipped face-up. (Flip Summon for monster and Activation for Spell/Trap cards, in general cases) However, face-up cards cannot be Flipped face-down, unless an effect specifically states so.
Monster Cards can be placed on the field in face-down Defense Position, generally by Normal Set (it is not possible for monsters to be in face-down Attack Position, post errata of "Darkness Approaches"). Some effects may Special Summon face-down, but only if a card states explicitly that it can do so. In some video games, card effects that Special Summon multiple monsters face-down also shuffle them on the same side of the field.
Face-down Monster Cards cannot be equipped with Equip Cards; if a monster equipped with an Equip Card(s) is flipped face-down, the Equip Card(s) equipped to that flipped monster is destroyed.
Link Monsters cannot be placed face-down.
Spell and Trap Cards can be Set in the Spell & Trap Zones or the Field Zone (in the case of Field Spell Cards). Quick-Play Spell Cards and Trap Cards cannot be activated during the turn they are Set. Pendulum Monsters cannot be played face-down in Pendulum Zones.
When a monster is flipped face-down, it will lose some specific information/applications on it [1] .
Some card effects can banish cards face-down. Cards that are banished face-down can be looked at by their possessor at any time, but cannot be looked at by their opponent. However, the number of banished cards (including face-down banished cards) is public knowledge.