Face-down

To say that a card is face-down, is to say the back part of the card is facing up. When a Monster card is face-down, it can only be horizontal. You cannot have a vertical face-down Monster Card.

When a Spell or Trap Card is face-down, it is always vertical.

When any card is face-down, unless specifically mentioned by a card effect, it's controller is not obligated to disclose any details of that card to his/her opponent.