The need to use a lining is related directly to the depth and sensitivity of a cavity and the nature of the restorative material. If a cavity is extremely shallow, e.g. the occlusal lock of a Class II cavity which has been cut to the amelo-dentinal junction and is not especially sensitive, no lining is indicated. On the other hand, if a cavity is deep, it will probably need a sub-lining of calcium hydroxide to protect the pulp, covered with a phosphate or polycarboxylate base. In a cavity for amalgam, a half- to one-millimetre depth of cement provides adequate insulation for the pulp. The technique for lining a Class II cavity for amalgam is as follows. If phosphate cement is to be used, it should be mixed to a putty-like consistency, so that a small pellet may be picked up on the point of a straight probe, rolled lightly between thumb and forefinger and carried into the cavity in such a way that it does not touch the walls. A suitable plugging instrument, such as a G plastic (Ash) or a Porro special A, is dipped in alcohol, the excess of which is shaken off, and the cement is packed against the pulpal floor, making certain that it does not fill the undercuts.