Commons
Commons are non-foil cards that are easy to obtain. The number of common cards per set of cards varies by the series.
Ways to trade:
1 Common for 1 Common
1 Common for 1 Story
2-3 Commons for 1 Energy
2-3 Commons for 1 Rare
3-4 Commons for 1 Foil
4-5 Commons for 1 Promo