A spile is a small wooden or metal peg used to control the flow of air into, and carbon dioxide out of, a cask of ale. It also can be used to obtain water from a tree.
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Ale use
Cask ale is served without externally supplied carbon dioxide or nitrogen. The idea that it is served "flat" with no bubbles at all, though, is a misconception. Properly conditioned cask ale contains bubbles of carbon dioxide generated by the natural action of the yeast living in the beer while it is transported and stored. In fact, the yeast will actually produce more carbon dioxide than is required, and it is for this reason that the cask is vented through the spile hole in the shive.
Two kinds of spile are available. Initially, "soft" or porous spiles are used, made of open-grained softwood, bamboo, or harder wood with cuts in. These allow the excess gas to escape. Once the cellarman judges this process to be complete, a hard spile is inserted which does not allow any more gas out of the cask. These are made of harder wood (usually still technically "softwood", but denser and more solid than soft spiles). Plastic pegs intended for sealing shives are occasionally used as hard spiles.
During service, the spile must be removed to allow air in to replace the beer drawn off. Once the spile is reinserted enough gas will come out of solution to replace what was lost, but the amount of carbon dioxide available in the system - free or dissolved - is limited, so it is important that the spile is kept in as much as possible. This is one of several reasons for real ale's short shelf-life once a cask is opened - if too much carbon dioxide is lost, the beer will be flat. Typically, the beer will be good only for two to four days; this short life is why it is important that a pub serving real ale have sufficient turnover for casks to be emptied while still at their best.
Water use
To get water from a tree with a spile, one must strike the spile into a tree to hit a xylem tube (not a phloem tube, because a phloem tube will bear sap); one must wait until the spile bears water.
Other uses for spiles
Like many such older terms, the word spile has other local meanings. For example: