The British Area Bombing Directive issued on February 14, 1942, focused on undermining "the morale of the enemy civil population and in particular the industrial workers". According to British philosopher A. C. Grayling, Lübeck, with its timbered medieval buildings, was chosen because the Royal Air Force (RAF) Air Staff "were eager to experiment with a bombing technique using a high proportion of incendiaries" to help them carry out the directive. The RAF was aware of using a high proportion of incendiaries during bombing raids was effective because cities such as Coventry had been subject to such attacks by the Luftwaffe during the Blitz. New heavy bombers, improved navigation and bombing systems, and new tactics led to a devastating increase in the effectiveness of the RAF's bombing offensive on Germany, starting with the bombing of Lübeck in March 1942. A series of follow-up attacks using a similar pattern, was mounted against Rostock between April 24 and 27, 1942.