Primary and secondary evidence are most easily discussed together. Primary evidence is sometimes also called "best evidence." It is the best evidence available to prove the fact in question, usually in an original document or record. The so‑called "best evidence rule" says that the highest possible degree of proof must be produced. The rule is realistic in its approach to evidence inasmuch as it does not ask the impossible, but rather looks at each question individually, on its own merits and circumstances. For example, it does not require evidence from a birth certificate to establish genealogical fact if the birth in question took place before birth registration. If, however, the primary evidence is from a document or record, rather than from first‑hand testimony of an eye‑witness, then the person who created that record (or who provided the information for its creation) must have been an eye‑witness to, or have had a special immediate interest in, the eventrecorded.
Secondary evidence is harder to define meaningfully. In essence it is all that evidence which is inferior in its origin to primary evidence (i.e., not the best evidence). That does not mean secondary evidence is always in error, but there is a greater chance of error. A copy of an original record provides secondary evidence, as does oral testimony of the record's contents. Published genealogies and family histories, as this definition should make clear, also provide secondary evidence.
Classifying evidence as either primary or secondary does not tell us anything about its accuracy or its ultimate value. This is especially true of secondary evidence. Thus it is always a good idea to ask the following questions:
1. How far removed from the original is it (when it is a copy)?
2. What was the reason for the creation of the source which contains this evidence?
3. Who was responsible for creating this secondary evidence and what interest did he have in its accuracy?
There are some situations where secondary evidence might be of even greater worth than primary evidence. These include those situations where the creator has carefully collected information from many sources (actually working with the primary evidence) into one place, such as a good family history.