Irregular chess opening

In chess, an irregular opening is an opening considered unusual or unorthodox. In the early 19th century the term was used for any opening not beginning with 1.e4 e5 (the Open Game) or 1.d4 d5 (the Closed Game). As opening theory has developed and openings formerly considered "irregular" have become standard, the term has been used less frequently.

Because these openings are not popular with chess players, the standard opening references such as Modern Chess Openings (MCO) and Encyclopaedia of Chess Openings (ECO) do not cover them in detail.