Prior to the 1960s dry paper was the main insulant in telecommunications cables. Paper with a very low density, and hence low permittivity, was very carefully slit to precise widths and lapped on the solid copper conductors. Twin insulated conductors, pairs, or four insulated conductors, quads, were prepared and made into units containing typically 25, 50, or 100 pairs, and these units were laid up to make 100 to 2,400 pair cables. The whole assembly was vacuum dried before application of a lead sheath to ensure that no moisture could enter the cable, and the electrical properties were maintained with dry paper and air as the dielectric. A laminate of PE and aluminum-tape sheath was introduced in the mid-1960s to avoid the high cost of lead sheathing, and some cables were made with paper-pulp insulation to increase production capacity.