I shall continue with my series of "Conventions you don't need to know". The 5NT opener is not a common beast, and you could be forgiven for thinking there is no such thing. Referring to various books about Acol I could find no reference whatsoever!
Fortunately, a couple of years ago I agreed to play with Paul Lamford in an event, and a detailed system discussion ensued. He told me that there was an obvious meaning to this opener - it asks partner to pick a minor, or bid a grand with a key minor suit card. "It's quite common," he assured me, "it's already come up twice".
As there is nothing terribly obvious apart from this, and it is easy to remember and use, it seems sensible enough. For example, with - A AQJ10xx AKQJxx you would open 5NT. If partner has the king of diamonds he would realise that that was a useful card and bid the grand. Similarly - - KQJ10xxx AKQJxx would open 5NT. It's no use opening 4NT on these hands - if partner showed two aces you wouldn't know which he had!
The system can also be abused a little. With - - KQJxxxxxxx AKQ you would also open 5NT. It is true that you haven't exactly got the minors, but you could safely correct to diamonds, and partner will get the message.
This is all very well, but what if you have a two suiter without both minors, and only one loser? I'm afraid you just have to rely on traditional methods, though I can give you some extra help. It is quite possible to play the 5NT opener as showing either the minors or the majors, with one loser. Partner assumes you have the minors, but only bids at the seven level if he has the necessary cards for both a minor and a major suit grand to make. If he just has the minor cards, he bids 6♥ or 6♠, telling partner to pass with a major, and bid on with a minor. And the great thing about this modification to the system is that we have doubled the frequency of the convention!
Now just sit back and await the arrival of this convention - though I can't guarantee that men won't arrive from outer space first!
♠ ♥ ♦ ♣ ♠ ♥ ♦ ♣
Addendum by JM: A great idea! I propose the following alternative: an opening 5NT should show any two suits, but missing precisely one of the four top cards. Responses are:
7♣: I have an A/K in three suits
6♥: I have an A/K in each minor
6♦: I have an A/K in ♣, but not in ♦,
6♣: I do not have A/K of ♣.
Over 6♣ or 6♦ opener can bid a suit he holds but in which he lacks an A/K. I think this enables us to cope with all eventualities, but only provided we are prepared to play in either of our suits. Cambs & Hunts website is clearly the place for up-to-date bidding theory debates!