Tschichold had converted to Modernist design principles in 1923 after visiting the first Weimar Bauhaus exhibition. He became a leading advocate of Modernist design: first with an influential 1925 magazine supplement; then a 1927 personal exhibition; then with his most noted work Die neue Typographie. This book was a manifesto of modern design, in which he condemned all typefaces but sans-serif (called Grotesk in Germany). He also favoured non-centered design (e.g., on title pages), and codified many other Modernist design rules.
Between 1947–1949 Tschichold lived in England where he oversaw the redesign of 500 paperbacks published by Penguin Books, leaving them with a standardized set of typographic rules, the Penguin Composition Rules. Although he gave Penguin's books (particularly the Pelican range) a unified look and enforced many of the typographic practices that are taken for granted today, he allowed the nature of each work to dictate its look, with varied covers and title pages. In working for a firm that made cheap mass-market paperbacks, he was following a line of work—in cheap popular culture forms (e.g. film posters)—that he had always pursued during his career.
His abandonment of Modernist principles meant that, even though he was living in Switzerland after the war, he was not at the centre of the post-war Swiss International Typographic Style. Unimpressed by the use of realist or neo-grotesque typefaces, which he saw as a revival of poorly-designed models, his survey of typefaces in advertising deliberately made no mention of such designs, save for a reference to 'survivals from the nineteenth-century which have recently enjoyed a short-lived popularity.'