Muiderberg is a bidding convention in the card game bridge. 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 also known as the Dutch Two, Lucas Two or Woo Two (mainly UK) opening.
The convention is named after the Dutch village Muiderberg, the residence of the designers of the convention - Onno Janssens and Willem Boeghem.
The partner of the Muiderberg opener can take the following actions:
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:
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 five cards or more in the bid suit (diamonds, hearts or spades) and four cards or more in a second suit which can be any of the remaining three suits.