Face-down means that the back part of the card is facing up, and the face of the card is facing down. This is the opposite position of the card being face-up.
The Main Decks and Extra Decks are placed on their Zone face-down at the start of every game. Cards are usually placed face-down via the act of setting.
Monsters on Monster Card Zone can only be face-down in horizontal position.[note 1]
Face-down cards have no name, Type, Attribute, Level/Rank, ATK, DEF and effect until they become face-up.[note 2]
^ Monsters can be in vertical position with the effect of "Darkness Approaches".
^ "Big Shield Gardna" can have its effect activated while face-down.