How does a whiskey decanter work?

This question usually gets answered as though a decanter were doing something to the whiskey. It is not. What follows is what actually happens inside one, which is more useful when you are deciding how to use the thing.

A ground glass stopper is made by grinding the stopper and the neck against each other with an abrasive until they mate. That produces a snug, individually fitted closure - which is why stoppers are not interchangeable between decanters, even identical ones from the same maker.

Snug is not sealed. There is no gasket and no compression, so vapor still passes. What the stopper does is slow the exchange between the air inside the decanter and the air in the room, which is enough to keep a bottle in good condition for weeks and not enough to keep one for years.

Alcohol is more volatile than water, so it leaves a partly open vessel faster. Losing a little alcohol would be a minor issue on its own. The real loss is aromatic: the volatile compounds that carry most of what you perceive as flavor are the first to go, so a whiskey that has sat in a loosely stoppered decanter for a long time tastes flatter rather than weaker.

This is also why the amount of air in the decanter matters. A decanter with two fingers of whiskey left in it has a large volume of air above the liquid, and that whiskey will fade considerably faster than a full one. If a decanter is getting low, decant less next time rather than topping it up.

For wine, a decanter works by exposing a large surface of liquid to air, which softens tannins and opens up aromas over an hour or two. For a distilled spirit at forty percent alcohol or more, that mechanism is not available in any meaningful degree - there are no tannins to soften and the spirit has already spent years in oak.

Anyone describing a whiskey decanter as letting the spirit breathe is applying a wine mechanism where it does not transfer. The full version of that argument is on are decanters worth it.

1. Decant a bottle you are actively drinking, not one you want to keep. Weeks and months are fine; years are not.
2. Keep it reasonably full. A near-empty decanter fades its contents faster because of the air above the liquid.
3. Keep it out of direct sunlight. Light degrades spirits over time and a decanter, unlike most bottles, is usually clear.
4. Check the stopper seats properly. If it rocks or sits high, it is not a matched stopper and it will slow evaporation less.
5. If it is leaded crystal, do not use it as storage. The lead research is the reason this is a rule rather than a preference.

For what to look for when buying one, see the decanter hub. For the vessel that is not a decanter, decanter vs carafe.