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The decanter is used for decanting wines. Sounds simple but in old wines, you are pouring the wine into the decanter very slowly to prevent any sediment from getting into the wine.
When decanting, pour the wine slowly until any sediment reaches the neck of the bottle. Doing so is the act of separating wine from the sediment. This is mostly in aged reds and older ports. It's ok to let the wine sit in the decanter and settle down.
Decanting young reds is very useful to let the wine open up. In fact, all wines will benefit from aeration into a decanter even from a few minutes.
The act of aeration into the decanter is what helps not just the wine sitting in the decanter but the wine in your glass as well.
For more on aeration, click here and watch our wine buyers discuss the benefits!