12. The doctrine of common purpose applies where a venture is undertaken by more than one person acting in concert in pursuit of a common criminal design. Such a venture may be described as a joint criminal enterprise. Those terms - common purpose, common design, concert, joint criminal enterprise - are used more or less interchangeably to invoke the doctrine which provides a means, often an additional means, of establishing the complicity of a secondary party in the commission of a crime. The liability which attaches to the traditional classifications of accessory before the fact and principal in the second degree may be enough to establish the guilt of a secondary party: in the case of an accessory before the fact where that party counsels or procures the commission of the crime and in the case of a principal in the second degree where that party, being present at the scene, aids or abets its commission(2). But the complicity of a secondary party may also be established by reason of a common purpose shared with the principal offender or with that offender and others. Such a common purpose arises where a person reaches an understanding or arrangement amounting to an agreement between that person and another or others that they will commit a crime. The understanding or arrangement need not be express and may be inferred from all the circumstances. If one or other of the parties to the understanding or arrangement does, or they do between them, in accordance with the continuing understanding or arrangement, all those things which are necessary to constitute the crime, they are all equally guilty of the crime regardless of the part played by each in its commission(3).