You can look at the previous installment of this series here. We will look carefully at the 2 Diamonds bid now.
To recap, we are looking at auctions in the family 1 minor - 1 major - 1NT.
2 Diamonds is the general purpose game forcing bid. The variety of hands that can use such a bid is too great to be described on a list, but we can make a list of the possible goals of the bid. Since opener is tightly defined in strength and distribution, in many cases responder will be able to simply bid a game, without any exploration. When he wants to get further information, these are the main possible reasons:
1. Finding a 5-3 major fit
2. Finding a 4-4 major fit in a yet unbid suit (this may include spades, depending on your partnership's agreements about the auction 1 Club - 1 Heart - 1 Spade. Check Part 2 of this series for a more thorough discussion).
3. Deciding between game in a minor and 3NT (because of extreme distribution, lack of adequate stoppers, or both)
4. Exploring slam possibilities
Opener's reaction over this bid is aimed at clarifying his major suit holdings to cater for the first 2 possibilities. If they are not in play, he describes the suitability of his hand for notrump or for higher contracts.
Let us look at a typical auction:
1 Club - 1 Spade
1 Notrump - 2 Diamonds
2 Hearts: any hand with 4 hearts. May have 3 spades also.
2 Spades: any hand with 3 spades. Denies 4 hearts.
2NT: any hand without 4 hearts or 3 spades.
Note how minimum or maximum does not come into play. I strongly recommend that you avoid using any other bid than those three. Some hands will make another bid attractive -- you may have an unusual hand, for example, a hand with 4 hearts and 5 clubs which is not strong enough to reverse. If you have Kxx AQJx x KJ10xx, a supermax with both 4 hearts, 3 spades, a nice five-card suit, an unexpected singleton, it looks like you should make some noise to announce this to your partner. However, the bidding proceeds more smoothly if the balanced (or, presumed to be balanced) hand stays out of the way of the unbalanced hand.
Responder's hand is more of an unknown quantity and he may have lots of features to describe. More importantly, he needs room to establish a suit at the 3-level to anchor any slam exploration. Suppose, for argument's sake, that you decide to use, say, a jump to 3 Spades to show that kind of "supermax, all great news" hand. Now you will need some artificiality to set a trump suit if you are interested in slam, since 4 of a major will not be forcing. There are 3 possible trump suits (spades, hearts, and clubs), but only 2 available bids below game (4 Clubs and 4 Diamonds)! It is not an unsolvable problem, but it is a headache that can be avoided if opener simply stays out of the way of responder's plan.
After opener bid something up to 2NT, responder will always have room to clarify his hand. Remember that he cannot have a true two-suiter if he bids 2 Diamonds, as discussed previously. Any new suit is a 4-card suit. Responder is unlimited, and so he can have a balanced hand looking for a 4-4 fit in a possible slam. For example, the auction 1 Club - 1 Spade - 1 NT - 2 Diamonds - 2 Spades - 3 Diamonds shows 4 cards in diamonds and some slam interest (else the bidding would have already ended in 3NT or 4 Spades). Remember that responder cannot have longer diamonds than spades if he has slam interest -- he would have responded 1 Diamond originally.
A final warning for those who are using this convention for the first time: beware of 1 Diamond - 1 Major - 1NT - 2 Diamonds. It is easy to forget that this is artificial, and this goes for both players. Responder may signoff in 2 Diamonds with a weak hand, and opener may pass 2 Diamonds, forgetting that it is artificial. This is a major disaster when it happens. So keep on the lookout that this does not happen to you. Remember, if responder wants to play in 2 Diamonds, the way to do that is to bid 2 Clubs, forcing opener to bid 2 Diamonds.