Abstract: A conically threaded closure system is described which provides a compressive radial force to axial force ratio in the range of 4.3-6.2:1, resulting in an optimum stress distribution throughout the thread structures, optimum thread engagement with minimal rotational effort and primary and secondary sealing means. The system comprises a container having a first conical thread structure and a cap having a second conical thread structure disposed on the inside surface of the cap skirt adapted to cooperate with the first conical thread structure. The first and second conical thread structures each have a conical angle in the range of approximately 20.degree. to 30.degree., preferably approximately 24.degree. to 28.degree..