A derivative source is defined as:
"materials that offer alternate versions of the original—typically transcripts, translations, abstracts, extracts, nutshells, indexes, and database entries. The best derivatives will preserve all the essential details of the original. Still, errors are frequent." (emphasis mine) 
Some of the common derivative works are cemetery surveys, marriage abstracts, deed abstracts and will abstracts. These can be in book form, published in a periodical, or in some sort of online database. I want to alert you to a specific trap that I don't want to you fall into when working with these types of sources. This trap usually involves books. It is easier to explain by giving an example.
One of the books I have in my private library is Marion County, Mississippi Miscellaneous Records. I like this book because it has all kinds of court abstracts. I especially like it because each entry has the book and page number of the court book it was abstracted out of. For example, on page 53 of this book you will see a will abstract for John Barnes, Sr. The compiler (E. Russ Williams) also documented that this will is in Marion County Will Book A, page 70-71. This will gives me all kinds of goodies; the name of his wife, his children, his grandchildren as well as "It is not the desire of John Barnes for Edmund Lowe to get any of his estate." (Ouch!)
So what is my source for the evidence contained in this will? Some researchers will cite Marion County, Mississippi Will Book A, page 70-71 and that is the trap. Your source is not the original will book but rather it is the book of abstracts, Marion County, Mississippi Miscellaneous Records. You can't cite the Will Book unless you actually viewed it yourself. The best-case scenario is to obtain a copy of the will from the will book so that you can analyze it yourself. If you do, you can then cite the will. If not, you need to cite the abstract book. This is what my citation would look like.
E. Russ Williams, compiler, Marion County Mississippi Miscellaneous Records (1986; reprint, Greenville, S.C.: Southern Historical Press, 2002), 53; citing Marion County, Mississippi Will Book A:70-71, John Barnes, Sr. will, signed 11 August 1838.
Did you notice that I snuck in the information about the will book? I did this so that my readers can have the benefit of knowing exactly where to go to get a copy of the will for themselves. They can see that I didn't actually consult the will but I am using an abstract book.
In tomorrow's Tuesday's Tip I am going to relate this to something you will see specifically in Legacy so stay tuned.
 Elizabeth Shown Mills, "Quick Lesson 17: The Evidence Analysis Process Map," Evidence Explained (https://www.evidenceexplained.com : accessed 12 December 2017).
Michele Simmons Lewis, CG® is part of the Legacy Family Tree team at MyHeritage. She handles the enhancement suggestions that come in from our users as well as writing for Legacy News. You can usually find her hanging out on the Legacy User Group Facebook page answering questions and posting tips.