In early 19th-century chess literature, all openings that did not begin with either 1.e4 e5 or 1.d4 d5 were classified as "irregular". As opening theory developed and many openings previously considered "irregular" became standard (e.g. the Sicilian Defence), the term gradually became less common. Opening books today are more likely to describe debuts such as 1.b4 (the Sokolsky Opening) as "uncommon" or "unorthodox".