There are three sources of European Union law: primary law, secondary law and supplementary law. Besides the case law of the Court of Justice, supplementary law includes international law and the general principles of law. It has enabled the Court to bridge the gaps left by primary and/or secondary law. International law is a source of inspiration for the Court of Justice when developing its case law. The Court cites written law, custom and usage. General principles of law are unwritten sources of law developed by the case law of the Court of Justice. They have allowed the Court to implement rules in different domains of which the treaties make no mention.