This is a theme that often surfaces in bridge discussion. I thought it would be useful to list the factors that would influence me to open 1NT or 1M, once and for all. At least this will allow me to simply point interested parties to this article! 

Why would anyone want to open 1NT when 1M neatly describes the main feature of the hand, the 5-card major suit, which is also a very live strain possibility? In a standard system (featuring 15-17 NT openings and a forcing NT response to the 1M opening), the main reason is that you will have a difficult time describing your high-card power if you open 1M. After 1M-1NT, new suits can be as weak as 12 HCP, and the 1M-1NT-2NT shows 18-19. Rebidding your majors with 5332 is a big no-no (partner will be misled as to both your strength and your suit length, expecting 6 cards there, if you do this!). The solution is to temporize with the cheapest natural new-suit bid. It is a bad kind of solution; no one feels comfortable rebidding Two Clubs in this auction with KJ1042 AQ5 K10 Q72. But it is your best bet if you decided to open One Spade and heard One Notrump in response.

On the other hand (bridge always has another hand), if partner does not reply to your 1M opening bid with 1NT, you are probably fine. If he raises you, you are certainly ahead of the NT openers (this is the worst case scenario for them). If he bids something else at the 2-level, you can rebid 2NT without misleading partner as to your strength.

This is not an easy situation, and if there were a perfect solution, it would be already widespread. Always opening 1NT will miss many desirable 5-3 fits. Never opening 1NT will lead to awkward auctions and missed games with the points divided 15-9 and the like. The best answer lies somewhere in the middle, but how to identify that middle? This is what this article is for.

We are assuming here that the question is whether to open 1NT with a 5332 hand containing a 5-card major, by the way. Devotees of the 1NT opening will do it with 5422 or even stranger distributions, but here we are not that creative (these hands will often have a comfortable rebid, eliminating the main issue with the 1M option). 

So, here we go, with a list of questions which you should ask before deciding. Remember, none of this criteria should be taken in isolation. If a hand fails a minor criterion but is overall fine for opening 1NT, do it.

1. Does the hand look notrumpy? That sounds vague but it is very important. A notrumpy hand has good intermediates, a preponderance of queens, a bad suit. Compare AK1098 AKJ 852 93 with J9852 KQ9 AJ KQ8. Your gut feeling should tell you which one is more suited to a 1NT opening. The borderline cases (AQ109x KQ10 J2 A103) will give you more trouble. Note how this second hand has a weak doubleton and a good suit, but it also has nice intermediates (that 10 of clubs can easily become a second stopper; but perhaps not. A109 would be better). When you get a borderline case, move to the next question.
2. Where is your doubleton? The ideal NT distribution has a doubleton in clubs. (Why do they always pick on clubs?). If the doubleton is in clubs, opening 1M means having to rebid 2D over a 1-level response, and 2D is always less suspect than 2C. Rebidding 2D with 3 cards exposes you far too frequently to an unwanted raise. The worst doubleton to have in a 1NT is in the other major, because this will often lead to a transfer from partner and perhaps a 5-2 fit when a 5-3 fit is available.  
3. What is your major?  Not all majors are created equal. Opening 1NT when you have hearts has a very clear advantage of preempting a 1S overcall. Opening 1NT with spades has less of an upside. 
4. What is your position at the table? 1st and 2nd position should skip over this question, but if you are in 3rd position, you should have more leanings towards opening 1NT. It is more preemptive by itself, and interference over a NT opening is much less precise than over a 1M opening. So, you are giving a tougher problem to the 4th bidder if he has a nice hand. Conversely, if you are in 4th position, lean towards opening 1M, especially with minimum hands, in which game requires a maximum from partner. After three passes, partscore bidding becomes more valuable.
   
5. Are you playing matchpoints or imps? Matchpoints places a higher premium in competitive auctions and in finding the best strain at all times, whether you have a game or not. Imp strategy involves bidding tight games and divulging less information in the process (to maximize lead and defensive errors). So, in borderline cases, 1M is to be preferred at matchpoints, 1NT at imps. 
6. Are you vulnerable? At imps, if you are vulnerable, the main goal is to get to a game if it is in any shape or form within reason. So, run for daylight and open 1NT in those borderline cases. In matchpoints this is a less pressing issue, so consider the other factors. 

Bridge literature can be technical and instructive, but it can also be fun. Some of the most amusing and fun books I have ever read (note that I did not say "bridge books") are presented below

Our suggestions for this week will be dedicated to the subject of percentage plays. This is an area of card play technique that may seem too stodgy for some, but it is essential to have a working knowledge of the odds to be an expert player.

After the opponents double our opening bid of 1 in a major, the whole context of our possible actions is affected. 

Reading good books is the best way to improve your game quickly. When it comes to declarer play, the more you see of any maneuver, the easier it becomes to identify it at the table. There is more to declarer play to that, but this is a fundamental step.

I recently hit upon a very clean, crystal clear example of how counting in defense can direct you to an unintuitive play.

When responder bid 1 Diamond, the entire context of the auction changes.​

You should be acquainted with  the articles on 2-way checkback Stayman  to fully profit from this article. 

The Part 3 of this series is here. Now we will wrap up the other bids available after 1m-1M-1NT.

You can look at the previous installment of this series here. We will look carefully at the 2 Diamonds bid now.