Maindeck

Your maindeck is the deck you actually start every game with, whether that’s the 60 cards of a Standard deck or the 40 cards you build in a Limited pod. It sits opposite the sideboard, which only comes into play between games in a best-of-three match.

Some players call the maindeck plus sideboard “the 75” in constructed formats, a shorthand for the full 60 cards you play with plus the 15 you can swap in.

What Goes in the Maindeck

A maindeck needs enough lands to hit your mana curve reliably, plus the spells that make up your core plan against most opponents. It’s built to be reasonably good in every matchup, since it’s the only deck you’ll have for game one.

Narrow answers, like cards that only matter against one specific strategy, usually live in the sideboard instead, where they can be brought in only when they’re actually useful.

Maindeck vs Sideboard

The maindeck is what you register and what you play with in game one, full stop. The sideboard is a reserve of up to fifteen cards you can trade in and out between games once you’ve seen what your opponent is playing.

This split only matters in best-of-three formats. In best-of-one, there’s no chance to sideboard, so everything you’d want against any matchup has to already be in the maindeck.

Maindeck Deck-Building on MTG Arena

Arena’s deck builder treats the maindeck and sideboard as separate lists, and Bo1 queues use only the maindeck since there’s no game two to sideboard into. That single difference changes how experienced players build for Bo1 versus Bo3 on the same platform.