Lettrism is a French avant-garde movement, established in Paris in the mid-1940s by Romanian immigrant Isidore Isou. In a body of work totaling hundreds of volumes, Isou and the Lettrists have applied their theories to all areas of art and culture, most notably in poetry, film, painting and political theory. The movement has its theoretical roots in Dada and Surrealism. Isou viewed his fellow countryman Tristan Tzara as the greatest creator and rightful leader of the Dada movement, and dismissed most of the others as plagiarists and falsifiers. Among the Surrealists, André Breton was a significant influence, but Isou was dissatisfied by what he saw as the stagnation and theoretical bankruptcy of the movement as it stood in the 1940s.

In French, the movement is called Lettrisme, from the French word for letter, arising from the fact that many of their early works centred on letters and other visual or spoken symbols. The Lettristes themselves prefer the spelling 'Letterism' for the Anglicised term, and this is the form that is used on those rare occasions when they produce or supervise English translations of their writings: however, 'Lettrism' is at least as common in English usage. The term, having been the original name that was first given to the group, has lingered as a blanket term to cover all of their activities, even as many of these have moved away from any connection to letters. But other names have also been introduced, either for the group as a whole or for its activities in specific domains, such as 'the Isouian movement', 'youth uprising', 'hypergraphics', 'creatics', 'infinitesimal art' and 'excoördism'.

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