General hardness (GH)
also: GH, total hardness
The calcium and magnesium content of brew water, the minerals that actively pull flavor from the grounds.
General hardness (GH) measures the total dissolved calcium and magnesium in water. These two minerals are the workhorses of extraction: they bind to flavor compounds in the coffee and help carry them into the cup, which is why brewing with near-zero-mineral water tastes thin and dull.
GH is one of the two numbers serious brewers care about, the other being alkalinity (also called KH or buffer). GH governs how much flavor the water can pull; alkalinity governs how much it tempers acidity. Tuning them independently is the whole point of water-recipes. The widely cited SCA target for total hardness is roughly 50 to 175 ppm as calcium carbonate, with around 68 ppm often quoted as ideal, though many home recipes push higher for extra extraction.
Magnesium and calcium are not interchangeable in taste: many enthusiasts favor more magnesium for brighter, more vivid flavors. GH is measured in ppm (as CaCO3) or in degrees, and is reported separately from hardness meant in the scale-forming, machine-damaging sense. See water-101 to start.