Muiderberg convention

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 a good suit of its own or with support for partner's opened 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 four clubs
• bid 3♦ with a minimum and diamonds / with four diamonds
• bid 3♥ with a maximum and clubs / with five or more clubs
• bid 3♠ with a maximum and diamonds / with five or more diamonds
• bid 3NT with 4-4 in both minors
• bid 4♣/♦ with a 6+ cards in the bid suit

Both approaches (2NT forcing to game or forcing for one round only) 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.

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.

• Antispades Twos

• Wilson, Gavin (2016). Antispades Twos — Open Markedly More Often. Claygate, Surrey, UK: Org2B Press. ASIN B01MRUJ8AA.