Definition:
An opening is irregular when the first move or two is played far less frequently than other openings. On White's first move, 1.g4 and 1.a3, among others, are considered irregular. After 1.e4 (which is not irregular!) 1...b6 and 1...f5 are irregular.
The motivations behind playing an irregular opening are:
- Since your opponent has probably not studied it, there is little chance that you will fall into a prepared variation.
- An otherwise well-prepared opponent might have trouble coping with the unfamiliar positions that arise.
The disadvantage of playing an irregular opening is that it has some logical flaw which makes it less attractive than other openings. For example, 1.a3 ignores the center and develops nothing. The move 1.g4 ruins White's Kingside.
The evolution of opening theory has reduced considerably the number of irregular openings. The move 1.b3, which was once highly irregular, has been played by some of the greatest chess grandmasters.
