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 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 Twos 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.
Another variant, called Antispades Twos, provides for either a two-suited hand containing at least nine cards in the two suits with four or more cards in the bid suit — clubs, diamonds or hearts — and four or more in another suit, or a single-suited hand of at least six cards in the bid suit, in a hand worth 5-10 HCP.