Dbl is penalty if by an unpassed hand.
2 (H or S) is natural and one-suited.
2 shows two suits of the same Color.
2 shows two suits of the same RAnk (both majors or both minors).
2N shows two suits of the same SHape (both pointed tops, Spades and Diamonds, or both rounded tops, Hearts and Clubs) -- but only by an unpassed hand in direct overcall position.
By a passed hand or in balancing position:
Dbl shows two suits of the same shape.