Magic: The Gathering is at heart a strategy game. Central to Magic's strategy is its resource system, based around cards called lands. A dual land is a specific kind of land that loosens resource restrictions by being more versatile than an ordinary, or basic land.
Magic's Resource System
Actions in Magic are given costs in mana, which comes in one of five colours: white, green, red, black, and blue. It can also have no colour, and is thus logically referred to as colourless. Mana is produced by a number of sources, but most commonly by cards dedicated to the purpose, lands. One land can be brought into play from a player's hand in each of their turns. These lands come in five varieties to match the colours of mana, and can be used once a turn (tapped in Magic parlance) to produce one point of the corresponding colour mana.
Or rather, basic lands come in five colours. In addition to the ubiquitous basic lands, which simply tap for one mana of a specific colour, there is a variety of non-basic lands that have other purposes. Dual lands are the simplest of these, being lands that tap for one mana of one of two different colours.
Use of Dual Lands
Dual lands may appear to the casual observer to be a little better than a basic land, but not the vast improvement they actually prove to be. This is for a number of reasons. The first is versatility. For exactly the same cost as a basic land, a dual land is considerably more versatile. Since most action costs require some of a particular colour of mana, a dual land that is, for example, blue-black, can be used to pay for actions requiring either blue or black mana. In the early game, when few lands are on the board, this is a considerable advantage over a basic land, and although this advantage lessens over the course of the game, it never disappears.
Another advantage concerns availability. Cards drawn into the hand are random, and thus it is possible that the selection of land available does not match the colours required for the other cards in hand. This occurrence is common and effectively paralyses the player. A dual land is only a single card draw but it plays the function of two. This is counterbalanced by the Magic rule that a deck may only stock four copies of any card that is not a basic land, but remains a powerful advantage.
Dual lands also provide a simple way for a deck to dabble in a colour. Rather than having to fill the deck with lands to support every colour that's needed, dual lands may be used to produce an extra colour of mana without extra cost. The amount available through four dual lands may be enough to use a few useful effects from a colour not otherwise represented in the deck.
Limitations on Dual Lands
So now I've hopefully convinced you that dual lands are great. Unfortunately, they are not quite as simple as what I described above. You see, the dual lands were printed in the original Magic set, Alpha, pretty much exactly as I described them; one card that is two kinds of basic land at once, with all properties of both. And they were good. Too good. The problem was that they were each strictly better than a basic land, and thus there was no reason besides availability to use basic lands instead of dual lands. (This was before the four-copy rule mentioned above) So any dual land printed since the original sets has some disadvantage to compensate for its versatility.
I will now describe the various types of dual lands that have surfaced over the course of Magic's history, along with their advantages and disadvantages
The Original (True) Dual Lands
None of the disadvantages of the original dual lands are intrinsic, rather they are imposed by circumstance. For one, they are not legal in most tournament formats, which generally track the most recent sets with varying degrees of closeness. Also, they haven't been printed since Revised Edition in 1995 and will never be reprinted; this drives their value up although they are nowhere near as rare as the Black Lotus.
One of the first variations tried was the so-called "pain land". These lands would only tap normally to provide colourless mana. They could be made to produce coloured mana, but in exchange they would deal a single point of damage to the player. As a player can only suffer 20 points of damage before losing the game, these lands made their dual nature more of a reserve power, something to be saved up for an opportune moment. Since, unlike almost all other dual land variants, they do not slow down the player's strategy in any way, they are very popular in current play.
The Mirage set brought the "fetch land" to the attention of Magic players everywhere. These cards did not produce mana by themselves, rather they allowed you to fetch cards that produce mana. The turn after a fetch land comes into play, it can be discarded to search the deck for a basic land of one of two specified types, and bring it into play immediately. This has the disadvantage of slowing down the player's strategy by forcing them to wait a turn before recieving the increased resources. It also has the disadvantage that once a colour of mana is chosen, the other becomes forever inaccessible since all that remains is an ordinary basic land, and it cannot be used to access colours of mana that are not already present as basic lands. However, basic lands have no further disadvantages and are not vulnerable to some effects that counteract or 'hose' non-basic lands like dual lands.
Onslaught contained a second variant of the fetch land. Rather than forcing the player to wait a turn, this variant deals one damage to the player when its ability to fetch a basic land is employed. This combines the fetch land's lack of future disadvantages with the speed of the pain land, and is considered by Magic R&D to be the most powerful dual land variant since the original.
Like the fetch lands, there are two variations that could be called 'slow lands'. The first comes from Ice Age, like the original pain lands. These slow lands are very slow, unusable the turn after each use. This is a considerable disadvantage, and as such these slow lands have never been repeated. Rather, Tempest fixed them by adding a non-slow ability to produce colourless mana.
More recently, Invasion introduced a form of slow land, the 'tap land', which is unusable on the turn that it is played and afterwards acts just as the original dual lands act. As minor as this disadvantage appears, it is enough. A turn's delay can be a significant disadvantage in the early game, and remains an annoyance in the late game. Notably, this makes these lands useless on the first turn.
Odyssey contained a series of 'filter lands' that are an interesting play on the dual land concept. Filter lands require a single mana of any colour to activate and produce two mana, one of each colour they are capable of producing. Thus, these require a free basic land to be activated and are otherwise inert, making them useless on the first turn. Also, they are incapable of producing only a single mana, and as unused mana results in damage uses for both of the differently-coloured mana points are needed. Nevertheless, they have the advantage that the mana used to activated does not need to be one of the colours that is produced, adding flexibility to a limited set of available lands.
Dual lands are an important part of the historical and current development of Magic: The Gathering. As a basic archetype for numerous variations, the dual land has appeared in many forms over the eleven years of Magic's existence. Maligned when they initially appeared, they have become fan favourites with the original ten being considered just below the Power Nine in desirability.
Sources include nine years of on-and-off Magic playing and http://magicthegathering.com/. Please /msg me if I've missed anything.
This writeup is copyright 2004 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.0/ .