The story of the creative relationship between the Dutch artist Piet Mondrian (1872-1944) and the British artist Ben Nicholson (1894-1982) is largely untold, yet during the 1930s they were leading forces of avant-garde art in Europe and their work was profoundly connected. This book accompanies an exhibition at the Courtauld Gallery, London, that will be the first to offer a comprehensive account of the parallel artistic paths charted by Mondrian and Nicholson during this remarkable decade. It will bring together an extraordinary group of paintings and reliefs to show how each artist was driven by a profound belief in the potential of abstract art to attain the highest aesthetic and spiritual power. Nicholson's visit in 1934 to Mondrian's studio in Paris, where they were both involved with the Abstraction-Creation group, inspired a friendship that would last until Mondrian's death ten years later and would see them working in neighboring London studios for two years from 1938. Together they contributed to a number of groundbreaking exhibitions and avant-garde publications, in which their work was often presented side by side, expressing their shared commitment to creating new forms of abstract art. The friendship spanned a highly productive period for both artists who, in their different ways, were determined to refine and intensify their abstract work. Nicholson found powerful confirmation of his artistic convictions through Mondrian's example, which emboldened his approach and led him to produce some of the very greatest works of his career. At the same time Mondrian was taking new directions in his painting, making greater use of expanded areas of white space in combination with restrained but intensive areas of vibrant color. He also renewed the possibilities of his famous horizontal and vertical black lines leading to his development of the `double line', which greatly enhanced the visual dynamism of his compositions and are recognized today as being among his most iconic painting. This publication will reunite paintings and reliefs which were exhibited or published together in the 1930s as well as the Mondrian works originally bought by collectors from Nicholson's circle. It will explore in depth the importance of the relationship and the shared artistic sensibilities of the two artists, shedding new light upon this vital episode in the history of abstract art.