Systems On After A Direct 1NT Overcall
It is fashionable among experts these days to play systems on after a direct 1NT overcall; I suspect most have not really thought about it. If they had, I believe that they would have concluded (as have I) that systems off is better. Two clubs as Stayman and Jacoby transfers are essential after your side opens 1NT, but there are four major differences between opening 1NT and directly overcalling 1NT. Let’s explore how these differences affect playing systems on after a direct 1NT overcall:
Opening lead considerations.
It is a disadvantage to have the lead coming up to a 1NT overcaller (as opposed to a 1NT opener) since opener’s partner will lead opener’s suit unless he has an even better lead (which opener probably won’t find)! If he does lead opener’s suit it will be through any card or cards that dummy may have in the suit, rather than the opener leading up to it or them if he is on lead (as in systems off). So, w.r.t. opening leads, it is clearly better to simply have the 1NT overcaller’s partner bid his suit naturally and have the lead come from the opener rather than his partner. Also, if you transfer (as in systems on) then opener can double to show a second suit and/or suggest an alternate lead, which he can’t do if you play systems off unless he is willing to risk bidding the suit at the three-level.
There is a cue-bid available.
The 1NT overcaller’s partner now has a cue-bid available for Stayman, unlike the partner of a 1NT opener. In addition to probing for a 4-4 major suit fit, the cue-bid can be used to handle strong one-suited or two-suited hands (see below for invitational hands) by starting with a cue-bid and then bidding a suit (rather than transferring into the suit), which is a forcing sequence. With a weak two-suited hand you would ignore your second suit (as you also would in systems on) and merely bid your best suit at the two-level.
A 1NT overcaller’s partner is more likely to have a bust than a 1NT opener’s partner, so you are more likely to be doubled for penalty.
If the opener’s partner doubles 1NT for penalty (or even if he merely passes), it is a serious disadvantage to be unable to run to two clubs or two diamonds (both of which are forcing in systems on).
Your side is less likely to have game-going or slam-going values.
So the importance of any advantages (if any) that playing systems on confers towards finding the best contract is lessened (especially at IMP’s), since usually only a part score is involved.
You can handle invitational bids in systems off by jumping in your suit rather than by transferring and then bidding 2NT or then bidding your suit (with six), as you would in systems on. A possible disadvantage to this is that, if you only have a five-card suit, you will play a 5-2 fit if partner declines your invitation and has a doubleton in your suit, whereas you are able to play 2NT using systems on. But, when this is the case, playing your 5-2 fit often works out better than playing 2NT.
There are several other popular conventions that have been insufficiently analyzed (or they would have become unpopular!). Examples: fast arrival, support doubles, requiring support for partner’s suit when using Drury or an immediate cue-bid, two-way new minor forcing, systems on after a 1NT opener is doubled for penalty, negative doubles after a 1NT opener, bidding a suit to show two of the top three honors after your partner has opened two clubs, and fourth suit forcing to game (as opposed to merely forcing to either 3NT or the four level). I cover these in detail in the section on conventions in my book, “Things Your Bridge Teacher Won’t Tell You” (Master Point Press).