A Single consists of only One card and can be either a suited card, or a Den card.
If a Single is a suited card, all other players must play a card of the same suit. The highest ranked card of that suit wins the Trick. If they do not hold a card of the same suit, they must play a card of the same rank if they hold one (this is known as Poaching.)
If they do not hold a card of the same suit or a card of the same rank, they may play any card they choose.
When a Trump suit has been established, the lowest ranked Trump card out-ranks the highest ranked suited card in the remaining suits.
If a Single is a Den card and a Trump suit has been established, the Den card is considered a Trump card and maintains its rank. All other players must first play a Den card of the same rank if they hold one. If they do not hold a Den card of the same rank, they must play a card of the same suit. If they do not hold a card of the same rank or a card of the same suit, they may play any card they choose. The highest ranked card of that suit wins the Trick.
If a Single is a Den card and No Trump suit has been established, all other players must first play a Den card of the same rank if they hold one. If they do not hold a Den card of the same rank, they must play a Den card if they hold one. If they do not hold a Den card, they may play any card they choose. The highest ranked Den card wins the Trick.
In No Trump, a Single Den card can only be beaten by a higher ranked Single Den card.
In No Trump, a Single suited card can only be beaten by a higher ranked Single card of the same suit.