Where is a base typically placed in dental preparation?

A base is typically placed on the floor of the preparation above the pulp chamber to provide thermal insulation, mechanical support, and to protect the pulp from potential irritation caused by restorative materials. By positioning the base in this area, it serves as an intermediary layer that absorbs stresses and minimizes the risk of post-operative sensitivity or complications related to pulp damage during and after the restorative process.

The other options do not accurately reflect the standard practice for base placement in dental procedures. For instance, placing a base directly on the pulp chamber is not advisable as it could lead to pulp exposure or irritation. Similarly, placing a base on the external surface of the tooth or at the gingival margin would not provide the necessary support and protection intended for the pulp chamber area, rendering them ineffective locations. Thus, placing the base on the floor of the preparation is the most appropriate technique.