A 1NT overcall is roughly equal to a 1NT opening in HCP. The vulnerability is the guiding factor. If vulnerable, the range is closer to 16-18. If not vulnerable, 15-17 is more likely. Of course, a stopper in the opponent's suit (especially if a major) is presumed. After our 1NT overcall, we pretend that their opening bid never happened. So, "Systems On" -- 100% "Front of Card."
|1||1NT||Pass||2=Transfer to Hearts|
If their responder acts after the 1NT overcall, we use the system we use for when they overcall our 1NT opening. [Example: (1) 1NT (2) -- Pretend we opened 1NT and they overcalled 2.]
In sandwich position, 1NT is natural/strong (many years ago, I played it as light takeout--but now, there are too many thieves out there and you need it as natural). So:
Our 2NT overcall (after their 2-level opening) shows a 1NT opening -- and is also influenced by the vulnerability. Of course, after a preempt, there are fewer options, so occasionally the 2NT-bidder could be off-shape. After our 2NT overcall, "Systems On." A 3NT overcall is natural, but very wide-ranging. I try not to remove a 3NT overcall without good reason, but if doing so, use 4=Stayman and 4/4 as Transfers (not the best system, but easy to remember, for the once a year that you'll use it).
A balancing 1NT is about a king less than a direct 1NT overcall. It is still "natural" and systems are still on. Even by a Passed Hand, 1NT in balancing seat is natural (about 10-11). Examples:
|1||Pass||Pass||1NT = 12-15 (approx) - natural|
|Pass||1NT= Nat (10/11)|
Our jump overcall of 2NT shows the two lowest unbid suits. So, after they open 1-of-a-Major, the jump to 2NT shows both minors. After they open 1, the jump to 2NT shows +. After they open 1, the jump to 2NT shows +. The suit quality is vulnerability-dependent (sounder if vulnerable). Always, at least 5-5 is promised. The bid is assumed to be preemptive; try to avoid it with "medium hands."
A jump to 2NT in balancing seat (for example: 1-P-P-2NT) is Standard. Just to make sure everyone knows what Standard is, that means 18-20, balanced and natural (no preempting in balancing seat). Of course, after such a rare animal, systems are still on. Balancing as a passed-hand with 2NT is unusual (two-suited) [Example: P (2) P (P) 2NT].
For a full LC Standard card and a prettier version of this series, see Bridge Winners.