Gertrude Stein, particularly in her writing after "Tender Buttons," and Louis Zukofsky, in his book-length poem "A," are the modernist poets who most influenced the Language school. In the postwar period, John Cage, Jackson Mac Low, and poets of the New York School (John Ashbery, Frank O'Hara, Ted Berrigan) and Black Mountain School (Robert Creeley, Charles Olson, and Robert Duncan) are most recognizable as precursors to the Language poets. Many of these poets used procedural methods based on mathematical sequences and other logical organising devices to structure their poetry. This practice proved highly useful to the language group. The application of process, especially at the level of the sentence, was to become the basic tenet of language praxis. Stein's influence was related to her own frequent use of language divorced from reference in her own writings. The language poets also drew on the philosophical works of Ludwig Wittgenstein, especially the concepts of language-games, meaning as use, and family resemblance among different uses, as the solution to the Problem of universals.