A wide diameter mouth allows easy drinking for session beers. It is easier to spill, though. A wide diameter mouth also spreads out the soft foamy head if a beer has high carbonation.
A narrow diameter mouth promotes sipping, which is appropriate for beers with bold flavor and aroma, or high ABV. A narrow diameter mouth concentrates aroma right at your nose.
An outward slope contours to lips for comfortable drinking. An outward slope supports a foamy head - the glass supports the foam from underneath, so they last longer. A mouth that flares out spreads beer broadly over your tongue. This increases sour flavor awareness and decreases bitter flavor awareness.
An inward slope increases the aroma that you perceive. The shape concentrates the rising aroma in a small opening right at your nose. Try covering your glass with your hand, swirling your beer, and then smelling at the mouth of the glass. A mouth that flares in directs beer straight back on your tongue. This increases bitter flavor awareness and decreases sour flavor awareness.
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