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A writer I adore posed the question in a forum, and the answer is important to know.
When it comes to copyrights, if you own the rights to your work, you own them for the duration of your life plus 70 years if it was created after January 1, 1978. You may will those rights to your heirs.
According to the U.S. Copyright Office, "For works made for hire, and for anonymous and pseudonymous works (unless the author’s identity is revealed in Copyright Office records), the duration of copyright will be 95 years from publication or 120 years from creation,whichever is shorter."
When you assign the right to your work to someone else – when they buy, outright, the rights to your work, in perpetuity, they keep those rights forever. All other cases of purchase or transfer of rights may vary, so be sure to have an attorney go over the legalese with you as you draft your will.
It is a little trickier when it comes to residuals, so on those it is best to consult with those who are paying the residuals and a lawyer. In some cases, however, even if someone else owns the rights to your work, if they are paying you some type of residual income for that, you may be entitled by law to will those residuals to your heirs, in perpetuity.
It is a good idea to create printed information for your heirs in regards to the work you own the rights to and the work you are paid residuals on even though you do not own the rights to it.
Information in regards to any other revenue programs, affiliate links, and the like will also need to be spelled out in writing to your heirs.
Remember to give as much information as you can so that your heirs can easily locate and access your work and your willed residuals – including magazine & book names, web site addresses, addresses, company names, writing work titles, log-in information, and account numbers.
You do not need to register your work with the copyright office, but it helps if you do. When you do so, you do not have to register each one separately. There is a fee involved, and you are allowed to register groupings of your work all at the same time so you only pay once for that entire group of work.
For more information, please refer to the U.S. Copyright Office or the office of the entity in your country that handles such matters.