In a classic case one of the best design for a post to decrease the risk of failure is the "narrowest and longest smooth, parallel post that one can fit into the post space". Utilizing the longest possible post ensures that the forces transmitted from the crown are distributed over as much of the root as possible. Using the appropriate diameter post ensures that as much natural tooth structure as possible is left to support the post and absorb the transmitted forces. A parallel post ensures the greatest retention of the post within the canal, and is perhaps utilized with only the slightest loss of tooth structure to the internal wall of the canal. A smooth-surfaced post transmits all the forces apically, which can cause fracture. Threaded or roughened posts, however, distribute the forces along the canal and are also more retentive than smooth posts. The ideal post is therefore parallel and threaded/roughened.