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The Muiderberg convention in the card game bridge is also known as the Dutch Two opening. It is a two-level preemptive opening based on a two-suiter with precisely a five-card major and a minor suit (four-card or longer). In Muiderberg the 2♥ opening denotes five hearts and an unknown minor suit, whilst 2♠ denotes five spades and an unknown minor suit.
The convention is named after the Dutch village Muiderberg, the residence of the designers of this two-level opening (Onno Janssens and Willem Boegem).
The partner of the Muiderberg opener can take the following actions:
- pass (with tolerance for the opened suit)
- bid 3♣ (a pass-or-correct bid)
- bid 3♦ to invite for game in the major suit
- bid 2♠ (over 2♥) as a contract improvement (opener is allowed to raise with a suitable hand)
- bid 3♥/♠ (opener's suit) as a preemptive raise
- bid 3♠/♥ (other major) which is non-forcing but invitational
- ask for the minor suit using a 2NT relay bid with a strong hand.
The 2NT response is often defined as forcing to game, for example in Biedermeijer and AcolPlus. In particular, with a strong hand with an own suit, responder must bid 2NT first as there is no other way to establish a force. The Muiderberg opener can respond in several ways to the 2NT answer:
- bid 3♣ with a minimum and clubs / with a four clubs
- bid 3♦ with a minimum and diamonds / with a four diamonds
- bid 3♥ with a maximum and clubs / with a five or more clubs
- bid 3♠ with a maximum and diamonds / with a five or more diamonds
- bid 3NT with 4-4 in both minors
- bid 4♣/♦ with a 6+ cards in the bid suit
Both approaches are equally popular.
In competition, 3♣ is natural (opener is expected to pass) while 2NT asks for opener's minor suit and is not necessarily strong. Further, 4♣/♦ are defined as fit-bids in competition.
A variant known as Lucas two-bids allow for 5 cards or more in the bid suit (diamonds, hearts or spades) and 4 cards or more in a second suit which can be any of the remaining three suits.