Muiderberg convention

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. NOTOC
The convention is named after the Dutch village Muiderberg, the residence of the designers of the convention – Onno Janssens and Willem Boegem.

Partnerships may choose to play Muiderberg Twos (or the variants below) as: (a) at least 5 4 shape, (b) at least 5 5 shape, or (c) at least 5 4 when non vulnerable but at least 5 5 when vulnerable, for added safety. Statistically, 5 4 hands are about four times as common as 5 5. 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. Particularly in the UK, the term Lucas Two is often used to describe what is actually a Muiderberg Two, where the second suit has to be a minor; this makes it safer for responder to explore for a second suit fit if short in opener's longest suit. A difficulty with Muiderberg and Lucas Twos is that it is no longer possible to make a Weak Two bid with a single suited major. These hands may instead be bid using the Multi 2 diamonds convention. 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.