E.5.1- Face down cards in the removed from game zone (see Rule Z.8) or phased out zone (see Rule Z.7) cannot be looked at by any player. [Onslaught Rules 2002/09/15]
E.5.2- Face down cards in play or on the stack may be looked at by the player that controls the card, but not by other players. [Onslaught Rules 2002/09/15]
E.5.3- If you control multiple face down cards or tokens in play, you must mark them or otherwise make it clear which is which. This includes, but is not limited to tracking the order the permanents entered play and which creature attacked last turn. Dice, tokens, and location on the table are common techniques for tracking face down permanents. [Onslaught Rules 2002/09/15]
E.5.4- Face down permanents in play are 2/2 (if creatures), have no color, text, expansion symbol, or name, have no creature type or other subtype, and have a mana cost {0}. Usually, the spell or ability that makes a card become face down will override the defaults by specifying at least some of the above, but if not specified these are the defaults. [D'Angelo 2002/09/19]
E.5.5- Face down spells on the stack are considered spells with the same characteristics the card will have when it enters play face down. See Rule E.5.4. [Onslaught Rules 2002/09/15]
E.5.6- Face down cards in play are revealed when they leave the in-play zone (except if phasing out), even if in the new zone the card would also be face down. They are also revealed at the end of the game. This allows everyone to ensure that the game was played fairly. [Onslaught Rules 2002/09/15]
E.5.Ruling.1- The spell or ability that turns a card face down will specify the requirements under which the card can be turned face up. For example, the Morph ability (see Rule A.29) requires you to pay the Morph cost as a requirement to turn it face-up while it is in play. [Onslaught Rules 2002/09/15]
E.5.Ruling.2- Turning a card face up does not trigger any "comes into play" triggered abilities. [Onslaught Rules 2002/09/15]
E.5.Ruling.3- Turning a card face up does not change its state, such as whether or not it is tapped, what effects or tokens are on it, whether it has "summoning sickness", or so on. Only the characteristics change away from the face-down definition. [Onslaught Rules 2002/09/15]
E.5.Ruling.4- If a face down card is copied, the current characteristics are copied. If the face down card was played with Morph (see Rule A.29), the copy does not get the Morph ability to turn face up. [Onslaught Rules 2002/09/15]
E.5.Ruling.5- Phasing in and out does not turn a face down card face up or otherwise reveal it. [Onslaught Rules 2002/09/15]
E.5.Ruling.6- Turning a face down creature card face up after blocking assignment will not undo any now-illegal blocking decisions. [Onslaught FAQ 2002/09/24]
E.5.Ruling.7- Only the card itself is face down. Any enchantments on it are still face up and counters on it are visible. [D'Angelo 2002/10/09]
E.5.Ruling.8- Face down creatures are not considered to have the Morph ability or any other ability. [Onslaught FAQ 2002/09/24]
E.5.Ruling.9- If a copy card is turned face down, its base characteristics are retained until it turns face up. If it had the Morph ability when turned face down, then that ability can be used to turn it face up. [Jordan 2003/02/16]
E.5.Ruling.10- It is possible for some changeable copy cards to be face down at the time they would be changed. If this occurs, the change modifies the face-down characteristics. For example, if a Vesuvan Doppelganger is copying a creature with Morph at the beginning of your upkeep, Backslide could be used to turn it face down before the triggered ability resolves. Then, when the ability resolves, it would take on the new form for its face-down characteristics. [Jordan 2003/02/16]
Note- Also see Morph, Rule A.29.
Note- Also see Illusionary Mask.