An outside prime is one which is built in the outer boards, and they are very rare beasts. In the above position, White has a full six- prime stretching from his 6-pt to his 11-pt. An inner board prime is mainly used to constrain your opponent's back men or, in the extreme case where the prime is actually the home board, to stop him from entering from the bar. Outside primes typically arise in complex holding games or back games. In the diagram position Black finds himself with four men behind a full prime and a useless home board - if he is lucky enough to hit a late shot he won't be able to contain the hit man.

If White redoubles, can Black accept the cube? He leads in the race by 91 to 113 but in reality he is a long way from a take. On each roll he will probably not be able to play some of his numbers. White can keep his 10 and 11-pts and build his board using the men from the 7, 8 and 9-pts. All the time he does this Black will not be able to play fives or sixes.

Also, when evaluating races be careful when you have more than two men on the 1-pt as this will create wastage - you will probably end up bearing off some of those men with 4s, 5s and 6s instead of 1s and 2s. The rule of thumb is to add two extra pips for each man (above 2) on your 1-pt and one extra pip for each man over 2 on you 2-pt. So in this position Black is at least 8 pips worse off than his actual count of 91 and things are likely to get worse.

Occasionally, Black will release a man only for it to be hit when White has a strong board. Jellyfish does not play outside primes particularly well so I treat its results with caution, but it has White winning 80 per cent of games with 14 per cent of them gammons. Even if it is slightly out this is still a very big drop.