Gaining access to materials of public international law is more difficult than to materials relating to the national law of single states. The greatest part of the publications relating to the law of a particular state is published within this state. The duty to deliver a depository copy, which exists in many countries, makes the publications relating to the law of this country available in the national library (arranged alphabetically and by subject). In addition, there is frequently a printed national bibliography, which makes these publications readily available.