What are the three parameters by which all beer styles are measured?
All beer styles are measured and categorized by their color, bitterness level, and alcohol content.
Beer color can range from pale straw to opaque black, depending on the style. This color can be measured on the Standard Reference Method (SRM) scale, anywhere between 1 and 40.
Bitterness in beer is measured in International Bittering Units (IBUs), a scale that can run from 1 to 100+, but the human palate cannot discern much differentiation in bittering past 70 or 80 IBUs.
Alcohol content, which is expressed as Alcohol By Volume (ABV), can be as low as 0.5% and as high as around 20% in a naturally fermented beer. Any beer that contains an ABV above this level did not achieve it by natural means, as normal beer yeast is incapable of fermenting that high.