When you choose to open 2NT, you show a balanced hand with 20 - 21 points. You might well have a five-card major (5-3-3-2). It is recommended to still open 2NT with this hand.
In case partner wants to know whether you have a five- or a four- card major, use a convention called Puppet Stayman.
Puppet Stayman is traditionally used over a 2NT opening (20 -21 points).
2NT p 3♣ (Puppet, artificial, asking if partner opened with a major, five or four cards).
Responses: 2NT p 3♣
3♥/♠ = a five-card major
3♦ = no five-card major, but a four (Responder now shows the four-card major they do NOT have, e.g. 3♥ = four spades, and 3♠ = four hearts)
3NT = no five or four-card major.
There are more answers to these bidding sequences, but this is plenty for now.
This same convention may be used over 1NT ( 3♣ = Puppet Stayman), and shows a hand which will play at least game. I recommend it, so that you use two Staymans, 1NT p 2♣ = normal Stayman enquiring about four-card majors, and 1NT p 3♣ = Puppet Stayman, enquiring about five-card majors. The latter is game forcing.
Puppet Stayman is used over 2NT (and 2♣ p 2♦ p 2NT p 3♣) openings.
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