The Dunst Opening, also known as the Van Geet Opening, Sleipner Opening, or Heinrichsen Opening, is an irregular chess opening defined by the move 1.Nc3. ECO code: A00.

History

The opening is named after Theodore Dunst (1907–1985), an American player who analysed and championed it, and also after the Dutch master Dirk Daniel van Geet (1932–2012), who played it extensively from the 1970s onward. It is classified among the irregular openings: while 1.Nc3 develops a piece, it does not directly contest the centre with a pawn and may obstruct the c-pawn's advance.

Main Lines & Variations

After 1.Nc3, Black's principal responses are:

• 1...d5 – The most common reply, occupying the centre. White can continue 2.e4 (leading to a reversed Scandinavian Defence after 2...dxe4 3.Nxe4) or 2.d4, transposing into standard queen's pawn structures.

• 1...e5 – Gaining central space. White may play 2.Nf3 transposing toward the Vienna Game or Four Knights Game, or 2.e4 reaching the Vienna Game directly.

• 1...c5 – A Sicilian-flavoured response; after 2.Nf3 or 2.e4, play often transposes into Closed Sicilian Defence lines.

Strategic Themes & Plans

White develops a piece without committing to a pawn structure, preserving maximum flexibility. The principal drawback is that 1.Nc3 blocks the c-pawn, limiting certain central formations. The opening can serve as a transpositional device into the Vienna Game, Closed Sicilian Defence, or various other systems. It has appeared sporadically at grandmaster level as a surprise weapon but is not part of mainstream opening theory.