Abstract: A plastic bottle mouth has a finish generally symmetric about a vertical axis of the bottle, suitable to receive a metal crown cap. The finish has a top planar surface lying in a plane normal to the vertical axis. A downwardly curved portion extends outward from the planar ring and terminates at an outermost edge located at a radius R1 bottle axis. An undercutting surface begins adjacent to the outer edge and extends axially inwardly and downwardly as a concave partial toroid. The undercutting surface can be defined in substantial part by a locus of points generated by a radius R having its origin on a circle of radius R2 lying in a plane normal to the vertical axis, particularly where R<<R2<R1. The radius R defining the undercutting surface is generally less than about 7% of radius R1, and can be only about 4% of radius R1. The planar ring on the top surface of the finish can have a width that is greater than about 2% of radius R1.