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)).