Accessory before the fact (A-BTF) is someone who intentionally helps P-1 beforehand, perhaps by obtaining the murder weapon or by encouraging P-1 to commit the murder, but is not present or nearby when P-1 commits the crime.
Accessory after the fact (A-ATF) is someone who, though not part of the planning or commission of the crime committed by P-1, intentionally renders aid after the crime. For example, he may furnish plane tickets to help P-1 escape or destroy evidence or hide the fruits of the crime. An A-ATF obstructs justice by making it more difficult to apprehend and convict the other parties to the crime. At common law husbands and wives could not be A-ATFs. Because of the marital relationship, they were expected to aid each other and therefore had an excuse if they did.
Individuals who, knowing that a felony had been committed, did not report it to authorities, could be convicted of misprision of felony at common law. A federal law enacted in 1908, 18 U.S.C. §4, makes misprision of felony a crime. However, it has been interpreted to require active concealment. A person cannot be convicted for simply not reporting the crime.
Only a few states recognize misprision of a felony, while a few states impose a general duty to report any known felony. Many states, however, impose a statutory duty on eyewitnesses to specified crimes to report them. And all states impose a duty on specified professionals (teachers or doctors, for example) to report suspected cases of child or sexual abuse.
All parties to treason were treated as principals.
All parties to a misdemeanor were treated as principals, though it was not a crime to be an A-ATF to a misdemeanor.
The MPC abandons the common law’s definitions of principals and accessories. It considers all actors, except those involved after the commission of the crime, as equals. Thus, §2.06 spells out the responsibility of principals in the first and second degree as well as accessories before the fact. The MPC provides a separate crime to cover the conduct of accessories after the fact.