In the Penedès we differentiate the two reasons for decanting a wine.
On the one hand, decanting prevents sediment reaching the glass. It must be done for very old wines which over the years have precipitated solids in the bottle, or in modern style unfiltered, very concentrated and tannic wines, which may also have precipitation in the bottle.
In these cases the wine should be decanted slowly, by tilting the bottle. It is very useful to do it with a light (a candle, for example) illuminating the bottle to see if the sediment is approaching the neck.
Another reason to decant a wine, is to help it breathe. Usually done with concentrated aged wines, which may require contact with oxygen to eliminate the reduction in the bottle and so open it up in all its nuances.
In very old wines with little color, be careful and it is advisable to decant a small amount first and see how it develops in the glass, as they are very delicate wines and abrupt contact with too much air after many years can spoil them.
To avoid sediment there are tall and narrow decanters, whilst wide low decanters are more appropriate for oxygenating wines as they have a larger surface area in contcat with the air.
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