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Irregular chess opening

Irregular openings are chess openings with an unusual or rare first move by White. These openings are:

The above openings are all categorized under the ECO code A00. Openings that are not "irregular" comprise:

If White plays a regular opening and Black responds in an unconventional way, the opening is not categorized A00. For instance, 1.e4 a6 is classified as B00 (King's Pawn Game).

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