In 1947, Klein began making monochrome paintings, which he associated with freedom from ideas of representation or personal expression. A decade later, he developed his trademark, patented colour, International Klein Blue (IKB). This colour, he believed, had a quality close to pure space, and he associated it with immaterial values beyond what can be seen or touched. He described it as ‘a Blue in itself, disengaged from all functional justification’. Klein made around 200 monochrome paintings using IKB. He did not give titles to these works but, after his death, his widow assigned a number to each one.