JJDD scoring is not complicated once you see it laid out, but the interactions — bids, bags, Nil, Blind Nil, the 10-bag penalty — decide games. Here's the full system in one place.
Bids and made bids
Bid X books, take X or more, and you get 10 × X points. Bid 4, make 4, that's 40 points. Bid 6, make 7, that's 60 points plus 1 point (bag) for the seventh book.
Bid X and take fewer than X and you get set: −10 × X. Bid 4, take 3, that's −40. Getting set is the fastest way to lose a game.
Bags and the 10-bag penalty
Every book you take beyond your bid is a 'bag' worth 1 point. Bags are fine in small numbers but poisonous in large ones. Accumulate 10 bags and you take a −100 penalty and your bag count resets to zero.
This is the bag trap. Teams that bid low every hand hoping to over-produce end up paying 100 points for the privilege every few hands. Good teams manage bags — bidding accurately, avoiding pointless extra books, and sometimes deliberately dumping bags on opponents.
Nil and Blind Nil
A Nil bid promises zero books. Make it (take zero books all hand) and it's +100 points. Miss it (take even one book) and it's −100. Your partner still bids normally, and their bid needs to be made separately.
A Blind Nil is a Nil called before you look at your hand. It pays +200 on a make and −200 on a miss. It's usually a comeback move when you're way behind — the risk math only works when you're desperate.
Winning the game
First team to 500 (or whatever target score you agreed on) wins. If both teams cross the target in the same hand, the team with more points wins — and if that's tied, you play another hand.
Games are won by making bids consistently, managing bags, and picking Nil spots correctly. Every rule above is just an implementation of one of those three ideas.
