Diamond Comic Distributors, Inc. (DCD) is the largest comic bookdistributor. They transport comic books from both big and small comic book makers, or suppliers, to the retailers. They distribute Marvel and DC Comics. DCD has a legal monopoly on comic book distribution. This monopoly is allowed because they don't have a monopoly on book distribution (books including non-comic books).
DCD has an "open-door policy" to new suppliers. This means that anyone who makes a comic book can send samples of it to DCD for review. If the comic book is of sufficient quality, DCD might distribute the comic book to retail stores for the comic book creator. Anyone with a comic book can apply for distribution by following the rules on DCD's website.