No deck is capable of beating every other deck it will ever come against. Every deck has weaknesses that make it vulnerable to certain types of spells. That can be used to your advantage, but it can also be used against you. That's why, when it comes to tournament play, your sideboard can be just as powerful as your main deck.
- What is a Sideboard?
Each player is allowed to bring 15 cards (in addition to his main deck) to a tournament. These cards are your sideboard. You're not required to have a sideboard; but, if you do have one, it must be exactly 15 cards. After the first and second game of a match, each player is allowed to swap cards in their deck for cards in their sideboard. This is a 1-for-1 swap, so both your deck and your sideboard must remain the same size. Your deck must still be legal after sideboarding (for example, if you already have four Llanowar Elves Buy in your deck, you can't sideboard in two more).
Sideboarding helps you address the weaknesses of your deck against a certain opponent. For example, if you consistently lose games against anyone who is playing red "burn" spells like Ghitu Fire Buy and Urza's Rage Buy, then you may want to put four Chill Buy or maybe four Ivory Mask Buy in your sideboard. Sure, you may still lose the first game; but after sideboarding, you should be able to hinder your opponent enough that you gain an advantage.
- Know the Metagame
So, now you know what a sideboard is and how to use it; but how could you possibly sideboard effectively against the hundreds of different deck-types that exist!?! That's where it becomes handy to be familiar with the metagame.
The "metagame" is basically just a fancy word for "the decks that are popular right now". At a tournament where there are 200 people, the chances are that there are not 200 different decks there. In fact, there are probably only 15 or so different decks (granted, each deck may have 50 variations, but that doesn't make it a different deck). Knowing ahead of time what these decks will be will significantly increase the effectiveness of your sideboarding.
So, how do you find out what the current metagame looks like? Go to your local game store and play a few different people, play in a few local tournaments, or even better, browse through the decks that have been posted here.
- Sideboarding Isn't Always Your Friend
There is a downside to sideboarding... your opponents get to do it too. So, you have to be prepared for it, regardless of how unfair it is.
Knowing the metagame doesn't just involve knowing what everyone else's deck looks like. You also need to know what their sideboard will look like. You may have the strongest green deck you've ever played, you may consistently win the first game of every match, but if all of your opponents have Perish Buy and Hibernation Buy in their sideboard, then you're probably not going to do too well in the second and third game.
You should remember to keep your deck versatile enough that these types of troublesome cards won't mean instant game-losses. In the previous example, you might want to consider playing a second color, or maybe changing your play style so that you constantly keep some cretures in your hand (thereby minimizing the damage caused by Perish Buy).