Trivia
Rules of Bridge "By the Numbers"

Rule of 2, 3 & 4:
Preempt such that you expect to loss two tricks with unfavorable vulnerability or three tricks with equal vulnerability and four tricks with favorable vulnerability. 
Rule of 7:
When declaring at notrump and determining how many tricks to hold up, subtract the number of cards that you and the dummy hold in the suit led from 7. Hold up that many times. Hence if spades are led and you have six spades in both hands, hold up one round and take the second spade trick. On the other hand, if you have five spades between you, hold up twice and take the third trick. 
Rule of 8:
When considering a twosuited overcall after a 1 notrump opening bid, if the number of losers you have (counting missing aces, kings and queens) subtracted from the number of cards in your two longest suits is no more than 2 and you have at least 6 high card points, (2 + 6 = 8 hence the rule name) then you may make a twosuited overcall with a fivefour distribution. (See ACBL Bulletin) 
Another Rule of 8:
Suit Quality counts in preempts. Add the number of cards in a suit + no of honors in that suit, if the sum is 8, preempt at 2 level, if 9 then preempt at 3 level. Special rules: When assessing suit quality (SQ), the jack and ten are counted only if the suit also contains a higher honor (ace, king or queen) but the suit has to be 6 cards long. J108643= SQ of 6: 6 cards but do not count the J/10 as the suit does not contain a higher honor. KJ8653= SQ of 8: 6 cards + 2 honors). The Suit Quality Test is useful when the quality of a suit is relevant to your bidding decision. For example, for a preempt you should have a decent suit. How decent? The SQ should equal the number of tricks for which you are bidding. Thus a weak two opening should have a suit quality of 8 (6card suit and at least two honors), a 3level preempt should have a suit quality of 9, and so on. (rtheb@videotron.ca) 
Yet another Rule of 8:
Mel's rule of 8: when to overcall over opponent 1 NT opening.Total number of cards in our 2 longest suits minus the total number of losers in your hand is 2 or greater = bid. Need at least 8 points in the 2 suits (4 at 3 level)
Example No 1
? K 5 3
? A Q 8 7 4
? K 7 2
? 9 3
Example No 2
K J 6 5 4
K J 6 5 3
8 6
5
Hand 1 = 8 cards in 2 longest suits but 7 losers ( 87 = 1 ) = pass
Hand 2 = 10 cards in 2 longest suits and 7 losers (107 = 3) = bidLosing tricks:

Count losers only in the first 3 cards of each suit ( the 4th., 5th., etc. cards in a suit are winners.

With 3 or more cards in a suit the Ace, K, Q are winners, anything lower are losers.

With 2 cards in a suit, Ace or King are winners, anything lower is a loser.

With 1 card in a suit, Ace is a winner.

There are never more than 3 losers in a suit


Rule of 9:
A guide to know when to double. Take the level of the opponent's contract (e.g. 4 hearts) and add the number of trump you have (e.g. 5 hearts) If those numbers total at least 9, then double. Attributed to Ray Depue 
Rule of 11:
To determine the number of cards in the partner of the opening leader's hand (or in the declarer's hand) of the suit in which the opening leader lead fourth down, subtract the number of the card led from 11. Reduce this count by the number of cards you see in your hand and the dummy that are higher than the card led, the result is the number of cards higher than the card led in the other opponent's hand. Note that you may find from this exercise that your partner did not lead fourth down. This is also a valuable insight. 
Rule of 13 for opening strong 2C:
To be used when you have one of those distributional hands with less than 22 points, and you are not sure whether to open one of a suit or 2C. Add up your defensive tricks (A=1, AK=2, KQ=1, Kx=1/2, QJx=1/2). Multiply the total by 2. Add all length cards of more than 3 in a suit. If the total is 13 or more, open 2C.For example, you hold: AKJxxxx KQJxx A void. Only 18 high card points, but what does the rule of 13 say? You have 2 defensive tricks in spades, one in hearts, and one in diamonds...total 4. Multiply by 2=8. Add length tricks (4 in spades and 2 in hearts)=14. Open 2C. If we change the hand slightly to AKJxxx KQJx Axx void, we still have the same 18 high card points, still the same void, still the same 4 defensive tricks, but now we only have 4 length tricks, so 4x2=8+4=12. Now this hand is not a 2C opener  submitted by txbigred.dorothymartin@verizon.net

Rule of 15:
When deciding whether to open light in fourth position, count the number high card points and the number of spades, if the result is at least 15, you may open. This relies on the principle that you do not wish to open the bidding for the opponents. If you control the spade suit, adequately, they will not likely bid over you. 
Rule of 15 for takeout X:
Count your HCP, add 6 for a void, 4 for a singleton or 2 for a doubleton in the enemy suit. Consider a X if the total is 15. as :
Spades: 3
Hearts: A Q 8 4
Diamonds: K J 4
Clubs: J 9 7 6 3
= 11 + 4 = 15 = X Shortest suit must be in enemy suit and must have 3 cards in any unbid suit. Contributed by Rene Theberge 
Rule of 16:
When deciding to raise a 1 NT opening to 3 NT, count the number of high card points and the number of cards 8 and greater. If the sum is greater than 16, you should raise to 3NT. This avoids the need for the use of 2NT as a bid showing 8 points and asking partner to raise if at the top of his 1NT bid. Accordingly, 2NT may be used for transfer or other purposes. 
Rule of 17:
If partner opens a weak 2, add the number of high card points plus the number of trumps you hold. If the sum is at least 17, bid game in partner's suit. (Zeke Grabour, ACBL) 
Rule of 20:
When deciding to open, you may open if the sum of your high card points and the number of cards in your two longest suits is at least 20. This rule is attributed to Marty Bergen. 
Rule of 21:
Ron Klinger has augmented the rule of 20 by also counting quick tricks and opening when the sum of the above and quick tricks was more than 21. (Max Hughes) 
Rule of 160:
If the ages of your opponents add to over 160, do not make the mistake of underestimating their abilities.