The same beer from the same brewery tastes different across two bars, and the cause usually sits between the keg and the tap. A draught system is a piece of equipment with several settings, and each one changes the pour.
Gas pressure sets the carbonation in the glass
Beer is stored under gas pressure, and that pressure has to match the carbonation level the beer was packaged at.
Too little and carbonation escapes into the headspace, leaving flat beer within a day of tapping.
Too much and the beer absorbs additional gas, pouring as foam and tasting sharp and prickly.
Line length and temperature control the pour
Beer loses pressure as it travels through the line, and the length is calculated so it arrives at the tap at close to atmospheric pressure.
A line that is too short delivers beer still under pressure, which breaks out as foam the moment it hits the glass.
Warm lines cause the same problem, which is why long runs from a cellar are chilled along their whole length rather than only at the keg.
Cleaning is the largest single variable
Yeast, protein and bacteria build up inside lines as a film that is not removed by rinsing.
That film contributes sour, buttery and vinegary notes to every beer passing through, and it affects delicate styles most.
Regular chemical cleaning on a fixed schedule is the difference between a bar where the beer tastes like the brewery intended and one where every tap tastes faintly the same.
Gas blends differ by style
Carbon dioxide alone suits most styles, dissolving into the beer and maintaining its carbonation.
Nitrogen dissolves poorly, so a blend containing it can push beer through long lines without over-carbonating, and produces the dense fine foam associated with stouts.
Serving a beer on the wrong gas changes its texture entirely, regardless of how well the rest of the system is set up.
The glass finishes the job
Residual detergent or grease prevents foam from forming and clings to the side as bubbles, which is visible as beer that leaves no lacing.
A properly rinsed cold glass produces a stable head, and that head holds the aromatics that would otherwise escape before the beer reaches the table.
Glass shape contributes as well, since a narrower opening sustains a head longer and concentrates aroma, while a wide mouth lets both disperse.
A bar serving one beer can get the glassware wrong without much cost. A list of twenty taps poured into a single shape flattens the differences the brewers built in.