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 2opening 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 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 3to 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 3with a minimum and clubs / with a four clubs
- bid 3with a minimum and diamonds / with a four diamonds
- bid 3with a maximum and clubs / with a five or more clubs
- bid 3with 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, 3is 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.