While abstract artists rejected the idea that art needed to be figurative to be beautiful, they still all used forms and shapes of some sort.
Klein went one step further.
He rejected the idea that an artist needed shapes altogether - and focused on the inherent power of colour.
Klein believed that beauty already existed in the world - his role as an artist was to identify and share it.
He wanted to "free colour from the carcan of form or shape" and "make the absolute visible".
For his first exhibition, Klein showed monochromes of different colours.
But the audience kept comparing the paintings with each other - which he didn't want.
So his next exhibition featured 11 identical blue paintings, priced differently.
He decided to stick to this colour and named it IKB - International Klein Blue.
Klein varied formats - canvasses, sculptures, sponges...
He would also invite others to paint for him - in some works naked models would cover themselves in IKB and brush themselves against the canvass, using their bodies as paintbrushes.
Klein's IKB paintings have become iconic.
One recently sold for $22 million.