Commonly used to refer to matters relating to the United Kingdom is the adjective "British". Northern Ireland remained part of the United Kingdom. Approximately 243,610 square kilometres is the area of the United Kingdom. Throughout the 20th century, coupled with economic decline after world wars, as other nations industrialised, the United Kingdom heavy industry declined and began to lose its competitive advantage. By 1961 this number, just over 0.7% of the United Kingdom population, had more than quadrupled to 384,000. In 2006 the United Kingdom was the largest puplisher of books in the world and in 2005, 206,000 books were published.