A 1NT or 2NT opener is presumed to be balanced or flat (no singletons, typically not 2 doubletons). Maybe once a year, I'd open 1NT with a singleton (click here to see an example). How about two doubletons? With an awkward hand such as:
I prefer opening 1NT to 1 (having an impossible rebid if partner responds 1). With 4 spades and 5-4-2-2 shape, I am okay with opening a 5-card minor (there is no rebid problem with 4 spades, since you don't have to worry about reversing).
Summary of Notrump Opening Bids: Almost always 4-3-3-3, 4-4-3-2 or 5-3-3-2. Rarely (only if stuck) with two doubletons. Once a year with a singleton (if really really stuck).
A 1NT response to 1 of a minor is usually balanced (but denies a 4-card major). When might the 1NT responder be unbalanced? Perhaps if responding to 1 with something like:
This hand is too strong to pass, but not strong enough for 2.
A 1NT response to 1 of a major is often unbalanced and can even include a void. For example, partner opens 1 and you hold:
There is no alternative to 1NT (too strong to pass, not enough for anything else).
A natural 2NT (or 3NT) response would promise a balanced hand (no singletons or voids). For example, respond 2NT (invitational 11-12) to 1 with:
Respond 3NT (13-15) to 1 with:
You wouldn't make this response with an unbalanced hand.
In competition, the notrump bid should also show a balanced hand. For example, if your partner opens 1, they overcall 2 and you jump to 3NT, you should not only have diamonds stopped, but a flat hand. Presumably you have 2 spades (with 3 you would have supported, probably via a 3 cuebid, and with a singleton or void, 3NT is not recommended).
Summary of Notrump responses: A 1NT response can be off-shape (unbalanced)--because it might be the only call available. A response of 2NT or 3NT guarantees flat (balanced) shape. Over interference, the notrump responder usually will not have any singletons or voids.
A rebid of 1NT or 2NT by opener should be balanced, although occasionally players make this call with a singleton in the suit in which partner responded. For example, you open 1, partner responds 1 and you hold:
Any rebid is a lie. 2 would show 6 or more. 1NT should be balanced. 2 is a reverse (definitely not strong enough for this call). So, perhaps the 1NT rebid with a singleton is the least-bad bid. In general, though, when opener rebids 1NT or 2NT he is showing a balanced hand.
Other Notrump Bids
Later notrump bids tend to show balanced hands, but occasionally the player bidding notrump will have no other suitable choice (pardon the pun). If there is shortness, it likely will be in the suit his partner has promised.
As South, I would have no alternative to 1NT, with say:
QUIZ on this topic.