A sound trademark is a trademark where sound is used to perform the trademark function of uniquely identifying the commercial origin of products or services. In recent times, sounds have been increasingly used as trademarks in the marketplace. However, it has traditionally been difficult to protect sounds as trademarks through registration, as a sound was not considered to be a 'trademark'. This issue was addressed by the World Trade Organization Agreement on Trade-Related Aspects of Intellectual Property Rights, which broadened the legal definition of trademark to encompass "any sign...capable of distinguishing the goods or services of one undertaking from those of other undertaking" (article 15(1)).