Mental Magic, which many players have called the most skill-testing format in the game, is a variant in which each player may play cards in their hand not as they are named but as any card with the same mana cost, however each card name may only be used once per game. For example, a card costing UU could be played as Boomerang, Counterspell, or Mana Drain, regardless of the actual card that it is. However, once one of these spells has been named, no other card may be cast as that spell for the rest of the game. Cards can be named in other places than in hand, as well. For example, although a creature spell once cast will remain the same creature as long as it remains in play, cards in the graveyard can be named for flashback purposes. A popular play is to use a 3U spell to cast Inspiration, and then afterward to use the same 3U spell in the graveyard to cast Deep Analysis upon oneself. This gains the player a great deal of card advantage, which is almost always paramount in Mental Magic. "187" effects, namely creatures with a comes into play effect, are another popular trick to gain card advantage. After one player has gained a crushing amount of card advantage he/she usually attempts to finish his/her opponent with a creature that is difficult to kill, such as one that cannot be targeted, is indestructible, can phase out at will, etc.
Rather than traditional constructed or draft decks, Mental Magic is often just played from a 'stack' of cards, that may simply be random or designed for this purpose. The two players may therefore play from the same deck, which is composed entirely of spells. The spells may then be placed face down on the field as land. These lands, sometimes referred to as "Utopia," can tap to produce one mana of any color and have the basic supertype, although they do not count as having any particular land type. For instance, a Utopia land can produce white mana and cannot be destroyed by Wasteland, but does not enable your opponent's creatures with Plainswalk.
Mental Magic also has its own banned list, which is composed of every card on the Vintage banned and restricted list, as well as some additions of cards that are considered to be too powerful given the mechanics of the format. For example, Trade Routes, which enables players to return lands to his/her hands, is often banned due to the fact that these "lands" can now be played as new spells. Additionally, cards that enable players to search their library for cards (Survival of the Fittest, for example) are often banned as well.
There is no one set of rules for Mental Magic. Due to the fact that cards that are playable from the graveyard are almost always extremely powerful (Flashback; Squee, Goblin Nabob; Haakon, Stromgald Scourge; etc.), many players ban naming cards in a graveyard altogether. Also, there are many different interpretations of how "search" abilities like Ramosian Sergeant should be handled.