A Prime is a row of spaces that is occupied by checkers and is blocking our opponent (or us). The effectiveness of a prime is determined by its length, and by the number of checkers that are blocked (or trapped) behind it.
According to the article on Possible Outcomes of Rolling Two Dice, all numbers between 1 and 6 have exactly the same probability: 1/18. Note that we are only interested in the number of each die and not in the sum of these two numbers. Namely, if roll 2 – 1, does not mean that we can jump over a prime that is 3 spaces long. Nonetheless, we can jump over a prime that is 2 spaces long when we roll 3, 4, 5 and 6. The probability for this is 4/18. Consequently, as the length of the prime increases, the probability to jump over it decreases linearly, e.g. 3/18 for a 3 spaces long prime, 2/18 for a 4 spaces long prime, and 1/18 for a 5 spaces long prime. From this analysis it is clear that we need to create a long prime. A 5-prime does look intimidating, however, maybe because it can become a 6-prime, but, strictly speaking we can save our checkers that are trapped behind it.
6 (or more) Spaces Prime
Things are getting really interesting when we have a 6 or more spaces prime. Now, the checkers that are behind that prime are literally trapped. They can not be moved at all. We (or our opponent) are in a bad situation, but, this does not mean that we lost the game. It all depends on how may checkers are trapped, and how easy is for our opponent to move his/her checkers.
Our first line of defense against a prime is to be proactive. There is no need to wait until it becomes a 5-prime. As soon as we see a 3-prime (or longer) forming, start moving the checkers that are behind it or occupy the space that is right after it. By doing this we achieve two things:
- We protect our checkers and thus we destroy our opponent strategy
- If we get lucky, we might build a prime in front of our opponent prime.
Another solution is to be aggressive, and start building our own prime. Depending on the game that we are playing this can be risky. We should attempt to build one, but more importantly we should pay attention to what our opponent is doing. Always have a copy of the article on probability to roll a certain number and progress accordingly.
As we wrote earlier, a prime is useful only if it is going to block a few checkers. There is no rule of thumb for this. We may win a game even if we block a single checker. Do not forget that our opponent can not start bearing off the checkers unless all of them are in the Inner Table. Once again, before we decide to build a long prime we need to evaluate the current situation. Maybe it is not in the best of our interest to build a long prime, and it is preferred to simply “run”, e.g. move our checkers as quickly as possible to the Inner Table.
Move a Prime
Is it possible to move a prime? Well yes, but it is tricky. We move it by blocking spaces that are in front of it, and releasing spaces that are behind it. We first block the front space, and we then release the space behind it. We may reverse this process, but, we should do this only if our opponent needs a certain combination of dice to jump over our prime, e.g. 1 – 6 or 2 – 6. We focus here on moving a 6 spaces prime. We have already discussed the smaller primes.
Priming in Game Variants
In most variants of the Backgammon game we need two checkers to occupy and own a space. Thus a 6 spaces prime requires 12 checkers, and then we have 3 more checkers to move freely, and use them to move that prime. Thus it is not that easy to build a 6 – prime is those variants, e.g Backgammon, Blockgammon, etc.
But, there are variants, like the Narde – Fevga game where with a single checker we occupy and own a space. In these games it is common to see a 6 (or more) spaces prime. Actually, there are case where a player can create two primes. In the Narde – Fevga variant a prime is extremely effective when it is created in our opponent’s Base Area. Because we are blocking him/her from moving checkers out from the Starting point. Another nice spot to create a prime is from point 24 until point 5. In the Narde – Fevga game, point 0 is the Starting Point (see Rules of Narde – Fevga).
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Enjoy the game!