The type of post that is chosen will depend upon a number of factors. Most posts used nowadays are prefabricated. The appropriate size is selected depending on the tooth.
A core is built from filling material around the part of the post that sticks out of the root after the post is cemented into the root. The core is shaped so that it will anchor and retain a crown.
Although posts are usually recommended when there is minimal support for a crown, they are not always necessary. The use of a post is determined on an individual basis based upon the support and structural strength required.