The Muiderberg convention in the card game bridge

Contract bridge

Contract bridge, usually known simply as bridge, is a trick-taking card game using a standard deck of 52 playing cards played by four players in two competing partnerships with partners sitting opposite each other around a small table...

 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

Major suit

In the card game contract bridge, the major suits are spades and hearts . The major suits are of prime importance for tactics and scoring as they outrank the minor suits while bidding and also outscore them...

 and a minor suit

Minor suit

In contract bridge the minor suits are diamonds and clubs . They are given that name because contracts made in those suits score less than contracts made in the major suits , and they rank lower in bidding. In particular, one can make game with a bid of 4 in a major suit, while a bid of 5 is...

 (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).

System

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
  Pass-or-correct bid
  In the card game bridge a pass-or-correct bid , is a non-forcing bid that asks partner to pass or bid differently based on her/his holding...
  
  )
• 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
  Minor suit
  In contract bridge the minor suits are diamonds and clubs . They are given that name because contracts made in those suits score less than contracts made in the major suits , and they rank lower in bidding. In particular, one can make game with a bid of 4 in a major suit, while a bid of 5 is...
  
  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.

Variants

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.