The best evidence rule is a rule in law which states that when evidence such as a document or recording is presented, only the original will be accepted unless there is a legitimate reason that the original cannot be used. This rule has its origins in the 1800s. With the advent of electronic communications, there has been some argument about how the best evidence rule should be applied, and whether or not it is relevant.
The idea behind the best evidence rule is that the best evidence is the original evidence. If someone writes a letter, for example, and someone else makes a copy, the copy may have errors if it was made by hand, or the copying process might somehow interfere with the letter. For example, a poor photocopy could obscure details of the handwriting, while a digital copy might alter the color and could also be manipulated by someone with skills.
For this reason, if it is at all possible, courts want to see the original evidence. If the original is destroyed or inaccessible for some reason, a copy will be accepted. However, the copy must be proved by a witness who can testify as to the contents and confirm that it is an accurate copy of the original. In the letter example above, for example, a lawyer might call upon the person who wrote the letter to ask if the copy is indeed a copy of the letter that person wrote, and to confirm that the copy has not been manipulated.
Electronic evidence can be trickier. For example, it can be difficult to identify the original copy of an email; it may be the copy in the sender's outbox, on the server, in the recipient's inbox, and so forth. Likewise, it can be impossible to tell if a file is an original or a copy, although it is possible to collect data such as when the file was generated and when it was altered. With digital evidence, courts may apply the best evidence rule in a case by case basis, depending on the evidence and the situation.
The best evidence rule was of special concern in an era when documents were copied by hand and when things like photographs could also only be reprinted through hand processes. Today, however, many pieces of evidence can be digitized in a very high quality form, and some courts may believe that these copies are every bit as good as the original. In addition, using digitized evidence in court allows people to secure the original evidence and to protect it from damage, loss, or theft.