The driving philosophy behind the Standard American bidding system (and most other bidding systems) is to show your shape first, then your points. You see, there are just too many different hands that can be made with 13 cards and not enough possible bids to show them all. So we tell our partner clues about our hand and hopefully come to the right level and strain of bidding using these clues and their negative inferences. So let's start with the opening bids.
For any balanced hand (4333, 4432, or 5332) we want to bid notrumps, but the range of points from minimum opening (12 points) to game points (25) is a huge range! If we were to split up that range into two bids, then 1NT would mean 12-18 and 2NT would mean 19-24. This makes it difficult for partner to know if the partnership has the requisite 25 points to bid 3NT. So we break it up into smaller ranges. Thus:
12-14 Bid longer minor, then bid cheapest NT (usually 1NT)
15-17 Open 1NT
18-19 Bid longer minor, then jump in NT (usually 2NT)
20-21 Open 2NT
22-24 Open 2C (strong hand alert!), then bid 2NT
With this ladder, you can see that you never get too high in the bidding and you limit your range to a two or three point range, making partner's job easier to pass or bid on to 3NT (or even bid higher into slam territory!)
Balanced hands are great because partner knows that if he or she has a 6-card suit, you automatically have a fit, because you can have at most one doubleton, and with a 5-card suit, it's a fair bet you have fit there, too.
If you don't fall in the balanced category, then your first priority is to show partner your longest suit. If you have 12-21 points, and one of your suits are longer than the others, bid it at the 1-level! Ok, simple enough, but what about if you have a two or three suits with the same number of cards? If they have 5 or more in each, bid the higher-ranking of the two (the ranks from high to low are Spades, Hearts, Diamonds, Clubs). If they are 4 card suits, bid the lower-ranking of the three (You can't have only two 4-card suits as your longest suits without being balanced.)
Here's a breakdown:
1C = 3+ clubs
1D = 3+ diamonds (usually 4+ but a 12-14/18-19 point 4-4-3-2 hand needs to be opened 1D)
1H = 5+ hearts
1S = 5+ spades
1NT = 15-17 HCP, balanced
2NT = 20-21 HCP, balanced
These bids will take care of all 12-21 point hands and show partner a major clue to the shape of your hand. All 22+ point hands are grouped into the opening 2C bid (which doesn't mean clubs, it just means monster hand.)
On your 2nd bid, you get to narrow your point range and tell partner more about your shape.
For instance, if you bid 1H, then on your second bid 2C, you are showing 5 or more hearts, and 4 or more clubs (that's 9 out of 13 of your cards!) Another example, you open 1D, then bid 2D, you are showing 6 or more diamonds (you neglected to show a second suit nor did you bid NT so the negative inferences suggest you are not balanced or 5-4) with a minimum range (12-16 points.) There are some "problem" shapes, though. It's tough to show a 4441 hand as opener, because you open with 1C or 1D, then your 2nd bid is usually a higher ranking suit at the 1-level, which partner might interpret as 5-4 (you usually bid the longer suit first.) Hey, no system is perfect and this is only a minor issue.
Let me introduce you to the single-most important idea in bidding: the Captaincy Principle. The Captaincy Principle works like this: if you know more than your partner does, you are the captain of the bidding. You decide how high and in what suit (or notrumps) you want to play. Example, partner opens 1NT. He has limited his hand to 15-17 points, and showed a balanced hand. You know a whole lot about his hand and he knows nothing about yours; thus, you are in control of the bidding. You may bid the appropriate level of NT or you can bid something else to ask partner to tell you more about his hand. That's where the Stayman convention, and Transfer Bids come in handy. Another example: you open 1H, partner bids 1S (6 or more points, 4 or more spades), then you bid 2H (showing 12-15 points, 6+ hearts). Partner knows more about your hand than you know about his, so he is the captain. Once you limit your range, the other person becomes the captain. Most conventional bids are made by the captain to inquire about specific characteristics of the other's hand. Until someone limits themselves, though, the bidding is a dialogue, a back-and-forth discussion about shape, looking for a fit in some suit.
As responder, you have point ranges, too. If you respond with a 1-level bid, you are showing at least 6 points, but you aren't limited. If you respond with 1NT, however, you limit yourself to 6-9 points, and your partner, the opener, is in charge. Since finding a major-suit fit is so important in Standard American, here's a breakdown of responses to 1H:
1S = 6+ points, 4+ spades
1NT = 6-9 points (no 4+ spades, no 3+ hearts)
2H = 6-9 points, 3+ hearts
2C/2D = 10+ points, 5+ cards in bid suit
2NT = 10-12 points, balanced (no 3+ hearts or 4+ spades)
3H = 10-12 points, 3+ hearts
Remember, you only need 25 points to bid game in NT or the major, so anything higher than 12 points as responder, you KNOW you're going to game and so it's relatively safe to bid higher. Notice I've broken the bids into what you can do with 6-9 points (minimum range to respond) and 10-12 points (invitational range.) This is basic, and there are lots of conventions that other bridge players will insist you learn in addition to these bids, but they aren't necessary. Also, notice that you only bid NT when you don't have a fit for partner's major nor when it's unlikely you have fit in the other major. After 1S, the responses are:
1NT = 6-9 points, no 3+ spades
2S = 6-9 points, 3+ spades
2C/2D/2H = 10+ points, 5+ cards in bid suit
2NT = 10-12 points, balanced (no 3+ spades)
3S = 10-12 points, 3+ spades
The major difference here is that you can't show the other major unless you have 10 or more points. The reason for that is you don't want to get too high in the bidding looking for a fit. What if partner opened a 12-point hand with 5 spades, 2 hearts, 3 diamonds, and 3 clubs and you have 6 points with 1 spade, 5 hearts, 4 diamonds, and 3 clubs? You might end up contracting for 8 or 9 tricks with no fit and only 18 points (the opponents have more points than you!) As I said before, no system is perfect and this is only a minor issue.
The only opening bids I neglected to discuss were the preemptive bids (weak hands with good trick-taking potential). I think I'll have to discuss those in another post. I hope that this post is helpful to those of you starting out and trying to figure why in the world you bid the way you do.
