The Atlas of the British Flora (Perring & Walters 1962) was a milestone in British botany, and most naturalists will be familiar with it as a basic reference souce for phytogeography and plant conservation for over 30 years. It showed for the first time the distributions of plants in 10-km squares across Britain and Ireland, allowing correlations of the distribution maps with environmental variables such as altitude, and indicating which species were common and which were rare.