Excerpts of copyrighted works may be copied as long as the excerpt does not exceed 10% of a performable unit. You can thus distribute copies of isolated portions but not complete movements. To proceed with a performance, you can create an emergency duplicate of purchased sheet music. This copy is called a "facsimile." A facsimile cannot be sold to provide income for its creator. As well, you are permitted to make limited numbers of photocopies of a single page of music for use by yourself or others in private study sessions. No more than four such copies may be made from any one page. The FBI may require you to sign affidavits confirming that you will destroy all original copies after making your copies. Sheet music can also be downloaded from the Internet. These files are in the same format as printed music and can be played on computer instruments capable of reading these files. They cannot, however, be printed directly from the web browser without further software installation. The copyright law allows certain activities involving copyrighted material. One such activity is "Performing Rights Society (PRS)." This system was established under U.K. copyright law for the purpose of assigning royalty payments due to performers and songwriters who have licensed their music for use in radio broadcasts. In the United States, PRS for Music has been adopted as an alternative method for determining royalties due musicians.