Dominaria United goes live on Magic: The Gathering Arena and Magic: The Gathering Online Sept. 1, followed by its paper release Sept. 9.
In effect, this gives Pioneer decks built around the five enemy pairs a built in advantage over allied color decks, since the enemy pairs have more lands to choose from when looking for mana sources.
The five colors form 10 pairs, and these 10 pairs are often separated into two groups — enemy pairs and allied pairs.
The last time pain lands were printed in a Magic Standard set was in the 2015 set Magic Origins, which only included the aforementioned enemy pairs.