Face down

Some cards (Illusionary Mask, Ixidron, and Tezzeret, Cruel Machinist), the morph and megamorph abilities, and the manifest keyword action allow spells and permanents to be face down.

A face-down permanent is, by default, a 2/2 creature with no rules text, no name, no subtypes, no expansion symbol, and no mana cost or color indicator (consequently, it is also colorless).

Face-down permanents put into the battlefield by Tezzeret, Cruel Machinist are 5/5 artifact creatures. Originally Illusionary Mask would cast the permanents as 0/1 creatures, but it was changed via Oracle to make it in line with morph.

Turning a face-down permanent face up is a special action that can be done any time the player has priority (in other words, any time he or she could cast an instant), but it doesn't use the stack and can't be responded to (in this way, it is similar to playing a land). To do this, it must either have morph or megamorph (and its controller pays its morph cost), or be a manifested creature card (and its controller pays its mana cost). A token, or a card that is already face up (such as a Clone copying a face down creature), can't be turned face up. If the face-down creature has any counters on it, those remain on the face-up permanent. It's not a new object, and it doesn't enter the battlefield just by turning face up. Any Auras or Equipment attached to it remain attached to it after it turns face up. There are three enchantments in Fate Reforged that manifest a card and then become an Aura attached to it.

Face down (together with its opposite, face up) is part of a permanent's status. Permanents have their own status, which is not a copiable value. Consequently, a copy of a face-down creature will be a face-up 2/2 creature with no characteristics except any that the copy effect gives it (e.g. Cryptoplasm). Since it's face up, such a copy can't be turned face up, unlike the object it's a copy of.

Cards may additionally be face down in other zones. This is the default for the library, but some effects also exile a card face down, and the cards in the special decks in the command zone for Planechase and Archenemy are also usually face down. None of these situations has any correlation with the "face down" status of a permanent.

Rules

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Face Down

1. A card is “face down” if it’s physically positioned so the card back is showing. Cards in some zones are normally kept face down. See section 4, “Zones.”

2. A status a permanent may have. See rule 110.5 and rule 702.37, “Morph.”

3. Face-down spells have additional rules. See rule 708, “Face-Down Spells and Permanents,” and rule 702.37, “Morph.”

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 708. Face-Down Spells and Permanents
  • 708.1. Some cards allow spells and permanents to be face down.
  • 708.2. Face-down spells and face-down permanents have no characteristics other than those listed by the ability or rules that allowed the spell or permanent to be face down. Any listed characteristics are the copiable values of that object’s characteristics. (See rule 613, “Interaction of Continuous Effects,” and rule 707, “Copying Objects.”)
    • 708.2a If a face-up permanent is turned face down by a spell or ability that doesn’t list any characteristics for that object, it becomes a 2/2 face-down creature with no text, no name, no subtypes, and no mana cost. A permanent that enters the battlefield face down also has these characteristics unless otherwise specified by the effect that put it onto the battlefield face down or allowed it to be cast face down. These values are the copiable values of that object’s characteristics.
    • 708.2b A face-down permanent can’t be turned face-down. If a spell or ability attempts to turn a face-down permanent face down, nothing happens and that effect doesn’t change any of its characteristics or their copiable values.
  • 708.3. Objects that are put onto the battlefield face down are turned face down before they enter the battlefield, so the permanent’s enters-the-battlefield abilities won’t trigger (if triggered) or have any effect (if static).
  • 708.4. Objects that are cast face down are turned face down before they are put onto the stack, so effects that care about the characteristics of a spell will see only the face-down spell’s characteristics. Any effects or prohibitions that would apply to casting an object with these characteristics (and not the face-up object’s characteristics) are applied to casting this object. The permanent the spell becomes will be a face-down permanent.
  • 708.5. At any time, you may look at a face-down spell you control on the stack or a face-down permanent you control (even if it’s phased out). You can’t look at face-down cards in any other zone or face-down spells or permanents controlled by another player.
  • 708.6. If you control multiple face-down spells or face-down permanents, you must ensure at all times that your face-down spells and permanents can be easily differentiated from each other. This includes, but is not limited to, knowing what ability or rules caused the permanents to be face down, the order spells were cast, the order that face-down permanents entered the battlefield, which creature(s) attacked last turn, and any other differences between face-down spells or permanents. Common methods for distinguishing between face-down objects include using counters or dice to mark the different objects, or clearly placing those objects in order on the table.
  • 708.7. The ability or rules that allow a permanent to be face down may also allow the permanent’s controller to turn it face up. Spells normally can’t be turned face up.
  • 708.8. As a face-down permanent is turned face up, its copiable values revert to its normal copiable values. Any effects that have been applied to the face-down permanent still apply to the face-up permanent. Any abilities relating to the permanent entering the battlefield don’t trigger and don’t have any effect, because the permanent has already entered the battlefield.
  • 708.9. If a face-down permanent or a face-down component of a merged permanent moves from the battlefield to any other zone, its owner must reveal it to all players as they move it. If a face-down spell moves from the stack to any zone other than the battlefield, its owner must reveal it to all players as they move it. If a player leaves the game, all face-down permanents, face-down components of merged permanents, and face-down spells owned by that player must be revealed to all players. At the end of each game, all face-down permanents, face-down components of merged permanents, and face-down spells must be revealed to all players.
  • 708.10. If a face-down permanent becomes a copy of another permanent, its copiable values become the copiable values of that permanent, as modified by its face-down status. Its characteristics therefore remain the same: the characteristics listed by the ability or rules that allowed it to be turned face down. However, if it is turned face up, its copiable values become the values it copied from the other permanent. See rule 707.3.
  • 708.11. If a face-down permanent would have an “As [this permanent] is turned face up . . .” ability after it’s turned face up, that ability is applied while that permanent is being turned face up, not afterward.

Exiled face down

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 110.5d Only permanents have status. Cards not on the battlefield do not. Although an exiled card may be face down, this has no correlation to the face-down status of a permanent. Similarly, cards not on the battlefield are neither tapped nor untapped, regardless of their physical state.

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 406.3. Exiled cards are, by default, kept face up and may be examined by any player at any time. Cards “exiled face down” can’t be examined by any player except when instructions allow it. However, if a player is instructed to look at a card and then exile it face down, or once a player is allowed to look at a card exiled face down, that player may continue to look at that card until it leaves the exile zone or is part of a pile of cards that are shuffled, even if the instruction allowing the player to do so no longer applies.
  • 406.3a A card exiled face down has no characteristics, but the spell or ability that exiled it may allow it to be played from exile. Unless that card is being cast face down (see rule 708.4), the card is turned face up just before the player announces that they are playing the card (see rule 601.2).
  • 406.3b Some spells and abilities allow a player to cast spells with certain qualities from among face-down cards in exile. A player may cast such a spell only if they are allowed to look at the face-down card in exile and if the resulting spell has the specified qualities.