Abstract: The neck of a container has a smooth seal surface engaged by the inner skirt or plug of a cap. Such surface is free of trim and parting line flaws which are characteristic of blow-molded jars because the surface is formed in such a way that the seal surface is not in contact with mold parting lines and further the trim (excess plastic) of the neck is located away from the sealing surface. In one form of the disclosure, above the sealing surface the neck wall slants outward and then bends upward-inward in a short, tapered stretch about 18.degree. to the horizontal which engages the underside of the cap disk to compress against the disk or a liner or foil. Other forms of seal surfaces are disclosed. The exterior of the neck is formed with shoulders which engage over locking beads on the interior of the outer skirt of the cap. Caps suitable for combination with the neck, methods of forming the neck and mold apparatus for forming the neck are also disclosed.