Used in agreed circumstances only, to define the odd/even lengths of all partner's suits suit in one hit, and therefore by a simple rule, also those of declarer.
Background and theory
“Parity” is defined as a suit length being either odd or even. Normal defensive methods commonly give partner your parity (“count”) in the suit being played, as this is often more use to him in defence than it is to declarer. Rather than just show the parity in one suit, prism signals show the parity in all suits in one go, and this can be of great help when a new suit is opened up before you have a normal count of that suit.
In a hand of 13 cards there will be 3 suits of the same parity, and the other suit will be of the other parity. That last suit is defined as the “prism suit”. The parity of this suit is defined as “prism parity”. The combination of the 2 defines the hand, so for example a shape of 5233 has a prism description of “even:hearts”. (Note the other 3 suits are odd parity). Similarly a 4333 is even:spades, and 4072 is odd:diamonds. A void is even parity.
Agreed circumstances and method
When the dummy goes down, work out and remember your own hand's prism description and also that of dummy. In the initial play you show your own hand type to partner while at the same time interpreting his description to you. How ? Often declarer will draw trumps when he gets in. When declarer does draw trumps, or if you lead trumps, both defenders show their prism parity and suit. This is the simplest circumstance.
If you have :
Zero trumps or One trump
The first discard shows the prism parity : an even discard is even prism parity, an odd discard is odd (the suit is ignored). The second discard shows the prism suit, in revolving style (eg if hearts are trumps, a high diamond shows spades, a low diamond shows clubs, a hi spade shows clubs, a low spade shows diamonds, a hi club shows diamonds, a lo club shows spades). Note that this discard shows the prism suit, it does NOT ask for a lead of that suit.
(You could alternatively use a McKenney/Lavinthal style to indicate the prism suit.)
Play hi-lo to show even prism parity, lo-hi to show odd prism parity. The first discard shows the prism suit, in revolving style. (If you followed hi-lo then your discard is meaningless, as your prism suit is obviously trumps.)
Three trumps or more
Play the first 2 cards hi-lo to show even prism parity, lo-hi to show odd prism parity. The third trump shows the prism suit. You have a choice of 3 cards to keep for the 3rd round to show this : the highest card last shows the prism suit to be the highest ranking, spades (or hearts if spades are trumps); the lowest card last shows the lowest ranking, clubs; and the middle card of the 3 shows the middle ranking of the other 3 suits. For example, hearts are trumps, and you have 972 ...
972 (in that sequence) is even:clubs, 927 is even:diamonds, 729 is even:spades
279 is odd:spades, 297 is odd:diamonds, 792 is odd:clubs.
If you are odd prism parity, you show your hand completely in the first 3 cards. If you are even:trumps then you have had to lie in showing your prism suit, so if you can, show it to be one you have already shown as odd by length signal - maybe on the opening lead – as this might expose the lie. (If you are even:not-trumps then the signal is pure.)
What if you have a trump honour?
If you have a picture in trumps then play naturally – PRISM IS OFF. However, your first 2 of 3 can still often show the prism parity. Partner's play is not affected, so you can still work out his hand.
If declarer fails to draw 3 rounds of trumps (the boss trump may be left outstanding in one of the defenders' hands or there may be no more trumps out) then in the first 2 rounds you have already shown your prism parity. Simple prism then abandons the showing of the prism suit. This information is still useful, because once length is shown in one suit by normal count methods, this may also define the others.
Other circumstances Prism can be used
You may agree to play prism signals on the run of a long suit from dummy in NT.
If only 2 rounds of trumps are played, you could make your next card (following suit) show the prism suit.
Play your normal signals (eg current length) at other times and before a prism is shown fully.
Finding declarer's hand
If partner has shown you his prism description and you remember yours and dummy's, then this gives declarer's.
The 3 known hands each have a prism suit and a prism parity. Combine the parities to get the collective parity, so for example even-odd-odd gives a collective even.
The suits will be all the same suit, or will be 2 suits (a pair the same and another) or 3 different suits.
Declarer's prism suit
If the known hands are all the same prism suit, so will declarer's.
If the known hands have 2 suits, a pair and a single, declarer's will be the same as the single, so making 2 pairs.
If the known hands are in 3 suits, then declarer's will be the fourth suit.
In summary, the 4 hands will all have the same prism suit, or there will be 2 pairs, or they will be all different.
Declarer's prism parity
This will be the opposite of the collective parity, except it will be the same parity when the hands all have different prism suits.
For example, if the 3 known hands are odd:spades, even:hearts, even:spades, declarer will be even:hearts
This can be useful if, for example, he ends in dummy and leads way from a suit headed by the king and you know he is odd in that suit, and (by the bidding) comparatively short. You may go up with the ace to prevent him sneaking a trick with a singleton queen.
Take an example. The bidding by opponents goes 1♠ 2♠ 2NT(general trial bid) 3♠.
When dummy goes down you see
You are even:hearts, dummy is even:diamonds.
Partner leads ♥ KAx and declarer follows with x, then the Q, then ruffs the third round. He draws trumps with AKQ, partner following 528, then leads ♦x from table. What do you play ?
Partner has shown even:hearts, so declarer is odd:diamonds. Declarer is therefore known to have had 6 spades, 2 hearts, and has therefore 5 cards in the minors. Specifically he has 1 or 3 diamonds. Declarer with long spades is more likely to be short in diamonds than partner, so you play ♦A immediately, felling declarer's Q.
and makes an overtrick if you don't go up with the ace of diamonds.
The prism signal enables you to find the correct defence.
Conversely, if dummy was the same but declarer had ♦ Qx, ♣ Axx and you went up with the ace, you lose a trick because declarer can then throw a losing club on dummy's ♦ K. With this hand you need to duck the first round of diamonds.
With this deal, partner would have shown odd:spades, so declarer is therefore odd:clubs. Therefore he has an even number of diamonds.
The prism signal enables you to find the correct defence.
Reliability, and is it worthwhile?
- If partner has a trump picture you are going to be misled, as for example he cannot signal the prism suit without perhaps losing a trick.
- If partner has 4 trumps and is even:trumps you may be misled.
- Opener may not draw trumps and the method cannot be applied.
Hands are rare that the method leads to an extra trick, as in the example above.