The raw materials for printing include paper, inks, and machinery. Paper is the most important material because it is the substrate on which all other printed materials are placed. It comes in many different types and grades, including groundwood (tree pulp), fine papers, special papers such as tri-fold brochures or flyers, envelope stocks and backing boards, bookbinding strings (for books with sewn bindings), glossy photo paper (smooth surface only), matte photo paper (textured surface only), transparencies (used mainly in advertising or promotional materials such as menus or posters).