Final answer:
A liner is placed on the dentin, just above the pulp cavity within the tooth, usually during restorative procedures like fillings to protect the tooth's nerve and promote dental health.
Step-by-step explanation:
A liner is placed on the dentin beneath the enamel, right above the pulp cavity within a tooth. When a tooth requires a restoration, such as a filling, a liner may be used to protect the pulp chamber and provide a therapeutic benefit to the deeper structures of the tooth. It acts as a barrier operating within the crown portion of the tooth, especially when the decay or damage is deep enough to be near the pulp but not extending into it. Dentists apply a liner to manage bacterial growth, insulate nerve endings to prevent tooth sensitivity, and to promote the health of the pulp.