Pouring wine from one container to another serves one of two purposes. It gets wine out of an unwieldy large bottle into two or more smaller ones, or it separates clear wine from sediment.
Older red wines are the ones that most often contain sediment. It is a mixture of tartrate crystals and color (all of the color in red wine is solid particles; with age, some coalesce and become too heavy to stay suspended). Sometimes whites will have deposits of tartrate crystals. They look something like ground glass, but are harmless and tasteless. In most cases, they are heavy enough to stay at the bottom of the bottle, but some hosts prefer to decant these wines, too.
Keep reading to learn how to decant wine.
To guarantee clarity to the last serving, sedimented wines are poured from their original bottles to a clean decanter or bottle in the following way:
Place a short candle or other small source of bright light next to the decanter. Holding the sedimented wine directly above the light, pour slowly into the decanter. Stop pouring when the light shows the first sediment reaching the neck of the original bottle.