In the law, primary evidence, commonly referred to as "best evidence," means the best, most authentic evidence available. Many common law systems use what is referred to as the "best evidence rule," which requires litigants to use only primary evidence in a case unless the primary evidence is unavailable. Within the United States, the primary evidence requirement has been codified into the "best evidence rule," found in rules 1001 to 1008 of the Federal Rules of Evidence.
The need to require original or primary evidence dates back to before there were computers or copy machines capable of making exact copies of a document. Prior to the advent of typewriters, and eventually copy machines, computers, and facsimiles, all copies of a document were made by hand. In some cases, a copy of a legal document was made by a clerk; however, in many cases, it was the litigant himself or herself that made the copy of the document being introduced into a court proceeding as evidence. It's not hard to imagine that this practice often led to abuses.
Within the United States, the "best evidence" rule still exists and requires that an original of a writing, recording, or photograph be produced when it exists. Although the rule exists, from a practical standpoint, most courts will accept a copy if the copy has been certified to be a true and accurate copy of the original by the clerk of court. England and Wales, as well as Canada, have all but done away with the rule by allowing all relevant evidence to be admitted.
In most cases, the party introducing the document bears the burden of proving to the court that it is a primary document in jurisdictions where the best evidence rule survives. If the original, or primary, document is not available, then the litigant must prove that fact to the court as well. If the court is satisfied that the original is no longer available, then a secondary document may be accepted into evidence.