Irregular Opening

Irregular opening is a traditional term for any opening considered unusual or unorthodox. In the early 19th century it 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 do not cover them in detail. Wikipedia
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