Seven pages on this site mention a glass without any of them explaining what it does. It is not a neutral container: it decides how much of the wine reaches your nose, and how long it stays at the temperature you chose.
Why a third is the right fill
Aromas are volatile compounds that must evaporate before they can be smelled. A glass filled to a third offers the largest liquid surface for the volume poured, and leaves the headspace those compounds need to gather. Filling to the brim does the opposite, and removes any possibility of swirling, which is simply a way of speeding up that evaporation.
The bowl gathers, the rim delivers
A wide bowl exposes more surface; a rim narrower than the bowl funnels what rises towards the nose. That is the whole mechanism, and it explains why an open bowl suits a reserved wine while a tighter one suits an exuberant aromatic style. The thickness of the rim matters more than most buyers expect: a fine rim lets the wine arrive without interruption, while a thick rolled edge changes how it enters the mouth.
The stem is not decoration
A hand holding the bowl warms its contents within minutes, which undoes the care taken to chill a white or to keep a red below room temperature. Holding the stem costs nothing and is the single easiest habit to acquire.
Grape-specific glasses deserve scepticism
Ranges designed for one variety are a marketing proposition more than a measurable one. The differences that survive scrutiny are size, rim diameter and thinness of the glass, none of which requires a different shape for every bottle. Two well-made sizes cover almost everything, including sparkling wines, which lose less aroma in a tulip than in a narrow flute.
Storage ruins more glasses than shape ever did
Detergent residue, a cupboard smelling of cardboard, or a cloth that has dried near cooking will contaminate a wine faster than any design flaw. Rinse with very hot water, dry with clean linen, and store upright: stored upside down, a glass traps stale air against its most fragile edge.
