Six-Point Primes

A prime is a row of consecutive points held by one player, each with two or more checkers. A six-point prime, also called a full prime, is the strongest formation in backgammon: any opposing checker behind it is completely trapped, because no die can carry it more than six pips. Once you have a full prime, the question is usually not whether you will win, but whether you will win a gammon.

Priming games in general are covered in priming games; this article is about the full prime.

Attacking with a full prime

To turn a full prime into a win, you need to bring it home without breaking it. For that you want spare checkers close to the prime. This is usually not hard, because the trapped opponent has few good moves. You can bring your remaining back checkers forward freely; even if they are hit, your opponent gains little while some of his checkers are still stuck behind your wall.

When your spares are in place, roll the prime forward. Take a checker from the back point of the prime and place it on the point just in front of the prime. Step by step, the whole block moves toward your home board and your opponent cannot stop it. Once the prime has rolled into your home board, the trapped checkers are closed out and you bear off as usual.

Play aggressively. Getting hit is rarely a serious problem, and sometimes it even helps: while you wait on the bar, your opponent may have to wreck his own position, since his trapped checkers cannot move. The real danger is running out of spare moves and being forced to break the prime. To avoid that, keep bringing your outside checkers toward the prime whenever you can.

A note on doubling

If your game is strong and you are likely to complete a full prime within a few turns, that is often the time to double. If you wait until the prime is built, you may be too late: your opponent will simply pass, and you give up your good chances for a gammon.

If you missed that moment and already have the prime, it is often correct not to double at all and to play on for a gammon. A position that is too good to double gives you more by playing it out. See playing for gammon for the math behind this.

Defending against a full prime

Being stuck behind a full prime is close to the worst situation in backgammon, so know what to do. Your goal is to hold on, keep a reasonable home board, and be ready for a shot when your opponent finally has to bring his checkers home.

• Do not hit if you stay trapped. If you hit but are still behind the full prime, your opponent may sit on the bar while you are forced to move your other checkers and ruin your own home board.
• Do hit if it breaks the prime. If hitting leaves him with only a five-point prime, it may be your best chance in the game.
• Hitting with a strong board can work. If you hold most of your home points, a hit may force him to waste time on the bar and break his prime first. This is rare and needs careful judgment.
• Do not leave a blot right in front of the prime. A blot there can be hit, then pointed on, then followed by more points while you dance on the bar.

Keep your trapped checkers together as an anchor at the edge of the prime and look to a backgame structure if you have several checkers back. Beyond that you need patience and some luck.

Conclusions

If you have a real chance to build a full prime, take it: it wins many games and many gammons. For the same reason, fight hard when you see your opponent's prime coming together. Attack, make points in front of it, and do whatever you can before the sixth point is made. The best defense against a full prime is not to let it be built.