This chapter describes some remarks on the making of parallel plates of high accuracy, as these can fairly be regarded as prisms of zero angle. Each face of the master prism is in turn set to zero in the telescope, and the face of the workpiece machined by the diamond lap. The prisms are machined oversize, cleaned and laid on the angular recesses with their lower ends bearing on metal strips. Each prism is put in a jig and the cathetus faces are machine milled on the glass milling machine. The prisms are blocked in plaster and the cathetus faces polished. The prisms are mounted with shellac at each of the four corners of the jig. The trued surfaces of the prism stacks are dusted and rubbed down into close contact with the tool surface. The prisms are finished by milling in accordance with whatever method of mounting the prisms is adopted in the instrument.