The player to the right of the dealer chooses a trump. When the hand is finished, that player deals the cards. Trump is passed from team to team and player to player this way.
In standard Jass, the player chooses one of the 4 suits as the trump suit. This suit is then stronger than any of the other suits in that game. Even if another suit is led (established), a player may elect to play a trump, even if they also have a card in the established suit. The strongest trump in a hand wins it. If there are no trumps in the hand, the strongest card in the established suit wins it.
However, in the trump, the Jack is the strongest, followed by the 9, with the rest of the cards in the same order as before. From strongest to weakest, the cards are:
Trump suit: Jack, 9, Ace, King, Queen, 10, 8, 7, 6.
If a trump is the established suit for hand, other players must follow with a trump, if possible. The only exception to this rule is the Jack of trump*. A player does not have to play this card. For example, the first 3 players in a hand have played the 6, 7, Queen of trump. The remaining player's only trump is the Jack. They wouldn't want to play it because while it wins the hand, the hand does not contain many points (see Card point values). Since the trump is the Jack, the player can throw a different card (see Strategies:Throwing away). If their only remaining trump was anything other than the Jack, they would have to throw that card, as with any other suit (see How to play a hand).
*Jack of trump is called 'Bur' (pronounced 'Boor') in Swiss German (phonetic spelling)
A more advanced form of Jass adds more choices for the player selecting a trump. The player may forgo using a trump at all and simply play all cards according to their normal strengths. Or the player may elect to use no trump, and reverse all the normal stengths of the cards. In both these variations, the strongest card in the established (first-thrown) suit wins the hand. From strongest to weakest, the cards are:
No trumps: Ace, King, Queen,
Jack, 10, 9,
8, 7, 6.
No trumps, reversed: 6, 7, 8, 9, 10, Jack, Queen, King, Ace.
Note: In Swiss German, the no-trumps mode is called Oben-abe (from top, down, pronounced 'Obenabe') and the reversed mode is called Unen-uffe (from below, up, pronounced 'Oonenoofe'). (Both spellings are phonetic)
If the designated player cannot choose trump, they can ask their partner to choose trump*. The teammate indicates their choice, but does not play the first card. The designated player then plays the first card in the chosen trump. If the player doesn't have trump, they must play a card anyway (see Strategies:If you have no trump).
See also Strategies:Choosing a trump.
*In Swiss German, one passes the trump by saying 'Gschobe' (pronounced 'Kshobe'). If the trump is decided in this way, it is called a 'Schieber' (pronounced 'Shee-eber'). This literally means 'passing over' or 'something that was passed'.