There are often emotional controversial discussions about the ideal closure for wine bottles. The point of contention is which system is the "right" one for which wine. This is directly related to the second question of whether and to what extent oxygen is needed for the development of the wine during bottle ageing or for shelf life. The air in the empty space in the neck of the bottle may be sufficient, but this is mostly filled with inert gas or vacuumed during bottling before sealing. Corks (natural corks) are up to a factor of 3 or 4 more permeable than alternative closures such as conventional plastic corks or screw caps.