As a general rule, we don't worry about "stoppers" unless the opponents have bid a suit. Otherwise, bidding notrump frequently describes a balanced hand (by the opener) or no better options (by responder).
Notrump openings and rebids
An opening bid of 1NT or 2NT shows a balanced hand (“standard” would be 15-17 and 20-21 respectively), but neither promises that all suits are stopped.
Opener’s 1NT or jump-to-2NT rebid also show balanced hands (12-14 and 18-19, respectively) but also don’t promise stoppers in all suits. For example:
I would rebid 1NT after 1
-1
with, say
J74 Q76 AKQ2 
J72.
I would rebid 2NT after 1-1 with, say:
A2 8764 AQ2 AKJ4.
In a 2/1 GF auction, I prefer to allow opener to rebid 2NT with a flat hand, but not everything stopped.
I would rebid 2NT after 1-2 with, say:
AJ765 KQ Q32 J76.
I would not rebid 2NT with a small doubleton in an unbid suit, preferring to raise partner with 3 trumps if need be.
1NT response:
A response of 1NT to 1 or 1 says nothing about stoppers. Responder might have several suits unstopped.
For example, I would respond 1NT to 1 with, say:
K6 765 J75 K7642.
But, if the opponents overcall, responder’s 1NT shows a stopper(s) in their suit.
For example: 1 (1) bid 1NT with:
KJ6 652 QJ6 Q764.
A response of 1NT to 1 or 1 (by partnership agreement, possibly “forcing” or “semiforcing”) says nothing about stoppers. Again, if the opponents overcall (such as 1 after 1), the 1NT response shows a stopper(s) in their suit.
Example:
1 P 1NT with:
86 62 QJ86 AJ764.
But...
1 (1) 1NT with:
AJ62 52 QJ6 10764.
2NT response:
Assuming it isn’t conventional (like Jacoby 2NT), a response of 2NT would tend to show the unbid suits stopped. If there is an overcall, then 2NT definitely shows their suit stopped.
Example:
1 P 2NT with:
Q106 K82 J86 AJ64.
1 (2) 2NT with:
KJ6 Q52 QJ6 Q764.
3NT response:
This should show a stopper(s) in all unbid suits (and definitely so after an overcall).
Example:
1 P 3NT with:
AJ6 KQ2 986 KJ64.
1 (1) 3NT with:
KQ6 K52 QJ62 K82.
If our side has bid two suits, natural bids in notrump tend to show stoppers in the unbid suits (but not guaranteed). If the opponents have bid a suit, then the notrump bidder is assumed to have that suit stopped. If stuck, you might have to bid 3NT naturally with an unbid suit(s) completely unstopped, such as:
1 (3) 3NT with:
1076 K2 AKQ106 Q54.
Or, (3) 3NT with:
AQ2 K J32 AKQJ106.
If your side has bid 3 suits, a notrump bid should deliver a stopper in the 4th suit. For example, after 1-1-2, a heart stopper would be expected for a notrump bid.
If the auction gets complicated, assume that the notrump bidder always has the opponent’s suit stopped. If he doesn’t, he shouldn’t bid notrump (sometimes a cuebid, often called a “Western cuebid,” can be used).
Conventional notrump bids (such as Unusual notrump, Jacoby 2NT) obviously say nothing about stoppers (nor anything about notrump shape).
For more, try this quiz.
Updated: February 2023