A co-author is a person who made a significant contribution to a work of authorship, that is a work protectable by copyright. Many works of fiction have two, and a few have three or more authors. The Mote in God's Eye by Larry Niven and Jerry Pournelle is a well-known example. Many scientific papers have several authors, Many popular songs have one author for the lyrics, and another for the music. Textbooks often have multiple authors. Three are many other common cases.
It is generally up to the primary author to decide who else has made a contribution sufficient to warrant making that person a co-author. In other cases, co-authors agree, sometimes in writing, on their roles. I myself have been a co-author on a scientific paper for which I did the computerized statistical analysis, while the senior author did the experiments and largely wrote the text.
In the US, any co-author may grant a license to a third party on whatever terms the co-author pleases, unless the co-authors have an agreement to the contrary. (The rule is different in the UK, where all co-authors must agree ro such deals.) The co-author must, however, account to the other authors for all receipts from any such license. In the absence of any agreement to the contrary, each co-author is entitled to an equal share of the profits, if any, of all sales, royalties, license agreements, and other profits from the joint work.
In those cases where the term of copyright is set as the life of the author plus some number of years, the death of the last surviving co-author starts the fixed term of years.
A derivative work is different. It is a new work in some way based on a previous work. (Examples are a translation, an adaptation, a sequel, a dramatization, a fictionalization, and a novelization.) The author of a derivative work does not become a co-author of the original work, nor does the author (or authors) of the original become co-authors of the derivative work.
The copyright owner(s) of the original must approve the creation of the derivative, and may charge a fee for this permission. Such a fee may be fixed, or be based on sales of or income from the derivative work. This is a matter of agreement, there is no default fee amount or structure for a derivative work.
In US copyright law 107 USC 101 defines a joint work:
A “joint work” is a work prepared by two or more authors with the intention that their contributions be merged into inseparable or interdependent parts of a unitary whole.
107 USC 101 also defines a derivative work:
A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications, which, as a whole, represent an original work of authorship, is a “derivative work”.
17 USC 103 (b) provides that:
(b) The copyright in a compilation or derivative work extends only to the material contributed by the author of such work, as distinguished from the preexisting material employed in the work, and does not imply any exclusive right in the preexisting material. The copyright in such work is independent of, and does not affect or enlarge the scope, duration, ownership, or subsistence of, any copyright protection in the preexisting material.
17 USC 106 provides in relevant part that:
Subject to sections 107 through 122, the owner of copyright under this title has the exclusive rights to do and to authorize any of the following:
(2) to prepare derivative works based upon the copyrighted work;
17 USC 201(a) provides that:
Copyright in a work protected under this title vests initially in the author or authors of the work. The authors of a joint work are coowners of copyright in the work.
17 USC 203 (a)(1) provides in relevant part:
... In the case of a grant executed by two or more authors of a joint work, termination of the grant may be effected by a majority of the authors who executed it; if any of such authors is dead, the termination interest of any such author may be exercised as a unit by the person or persons who, under clause (2) of this subsection, own and are entitled to exercise a total of more than one-half of that author’s interest.
17 USC 302(b) provides that:
(b) Joint Works.—In the case of a joint work prepared by two or more authors who did not work for hire, the copyright endures for a term consisting of the life of the last surviving author and 70 years after such last surviving author’s death.
Other Sources on US law
The article "Copyright Ownership: The Joint Authorship Doctrine" from Fndlaw reads in relevant part:
The Rights and Duties of a Co-Author
If the work qualifies under the law of copyright as a work of joint authorship the co-authors or collaborators may allocate the rights and duties of the work of authorship among themselves. However, since no formal agreement is required between the co-authors or collaborators a legal relationship of joint authorship may occur even without the intent of the respective authors to create a work of joint authorship. Therefore, if no joint authorship agreement has been formalized it will be presumed that the following principles shall apply.
Each co-author will own an equal ownership share in the work. This will occur even if one of the co-authors has contributed a greater quantity of the work than the other co-authors.
Each co-author will own an "undivided" interest in the entire work. This means that if the publishing project consists of illustrations and text that the artist and the writer will each own fifty percent of the entire work, i.e., the art and the text.
Any co-author, without the permission of their fellow co-authors, may grant non-exclusive rights to the work to third parties. However, a co-author may only grant exclusive rights to the work to third parties if the co-author obtains the prior consent of the other co-authors.
Each co-author has a duty to account to the other co-authors for any profits obtained from the exploitation of the work.
A co-author has the right to assign his/her ownership share in the work to a third party or to bequeath his/her ownership share to his/her heirs.
Each co-author will be entitled to equal authorship credit for the work upon its publication.
The policy document Joint authorship and collective works from the University of California reads in relevant part:
A joint work is a work prepared by two or more individuals, with the intention that their separate contributions be merged into a single work. A joint author can also be an organization or a corporation under the definition of "work made for hire." A person who has merely contributed ideas without actually documenting those ideas generally cannot be considered an author for purposes of U.S. copyright law.
Co-authors own the work’s copyright jointly and equally, unless the authors make an agreement otherwise. Each joint author has the right to exercise any or all of the exclusive rights inherent in the joint work. Each author may:
- Grant third parties permission to use the work on a nonexclusive basis without the consent of other joint authors
- Transfer their entire ownership interest to another person without the other joint authors' consent
- Update the work for their own purpose
Additionally, each joint author must account to the other joint authors for any profits received from licensing the joint work, though such profit accounting could be altered through an agreement among joint authors.
Collaborators should try to clarify joint ownership interests in a written (or even an oral) agreement, covering such issues as:
- ownership and use
- rights to revise the works
- marketing and sharing of any revenue
- warranties against copyright infringement
At UC, the ownership of joint works is determined by assessing the category of work for each of the contributors, as described in Section III.B of the UC Policy on Copyright Ownership.
As explained on “What do I own?,” copyright protection is automatic for any "original work of authorship" created and "fixed in any tangible medium of expression." From the perspective of copyright law, an author is generally someone who creates that expression, such as the artist who drew a picture, the musician who composes the notes, or the writer who composes a short story. Authorship under copyright law is a separate determination from the concept of authorship in scholarly publications like research articles, which may be based on disciplinary norms about credit. Such norms may instead consider things like who designed an experiment, who did the laboratory work the article is describing, or who leads the laboratory where the research took place. Those factors, by themselves, are not necessarily sufficient to establish authorship (either sole or joint) under copyright law.
"Collaboration, Co-Authors, and Joint Authorship: Who Owns the Copyright?" from Res Nova Law reads, imn relevant part:
The U.S. Copyright Act provides the default rules for who owns what when it comes to joint works, or collaborations between two or more authors. As stated above, co-authors are co-owners of a joint work. This means that each co-author is entitled to an undivided interest in the work as a whole. In other words, each co-author owns the copyright equally, regardless of the extent of their authorship. For example, even if you did 95% of the work and your collaborator only did 5% of the work, you still share equally in the ownership of the copyright. Having an undivided interest in the joint work means that each co-owner can license or transfer their interest to another without permission from the other co-owner(s), and each have independent standing to sue for copyright infringement without needing to join the other co-owner(s). See Sybersound Records, Inc. v. UAV Corp., 517 F.3d 1137 (9th Cir. 2008). However, each co-owner is subject to a duty of accounting to the other co-owner(s). That is, a co-owner who unilaterally exploits the joint work must account to the other co-owner(s) for profits derived from such exploitation. See Goodman v. Lee, 78 F.3d 1007, 1012 (5th Cir. 1996).
The Copyright Act's default rules come into play when the co-authors do not have a contract stating otherwise. However, you can always draft your contracts around the default rules. For example, if you do not intend for each co-author to have an undivided interest in the work under the default rules, then you'll have to draft very clear contract terms ...
Rights and licensing of joint works in Canada indicates that in Canada, unlike the US, all co-authors must consent to any licensing arrangement. However this is based on "exactly one case, in a court of first instance, with no citation or in-depth reasoning as to why consent of all joint authors is required." The case cited is Pinto v. Bronfman Jewish Education Centre, 2013 FC 945 sand the answersuggests that:
Canada's joint copyright regime is a joint tenancy regime like that of the UK
as opposed to
a US-style tenancy-in-common approach to joint copyright.
Section 9 (1) of the Candian Copyright Act of 1985 provides that:
9 (1) In the case of a work of joint authorship, except as provided in section 6.2, copyright shall subsist during the life of the author who dies last, for the remainder of the calendar year of that author’s death, and for a period of fifty years following the end of that calendar year, and references in this Act to the period after the expiration of any specified number of years from the end of the calendar year of the death of the author shall be construed as references to the period after the expiration of the like number of years from the end of the calendar year of the death of the author who dies last.
Section 13(3) of the act provides:
(4) The owner of the copyright in any work may assign the right, either wholly or partially, and either generally or subject to limitations relating to territory, medium or sector of the market or other limitations relating to the scope of the assignment, and either for the whole term of the copyright or for any other part thereof, and may grant any interest in the right by licence, but no assignment or grant is valid unless it is in writing signed by the owner of the right in respect of which the assignment or grant is made, or by the owner’s duly authorized agent.
But this section does not address the case of joint authors.