Mineralogy
"Hardness" is determined by comparison with other minerals. In the Mohs scale, a standard set of minerals are numbered in order of increasing hardness from 1 (talc) to 10 (diamond). A harder mineral will scratch a softer, so an unknown mineral can be placed in this scale by which minerals it scratches and which scratch it. A few minerals such as calcite and kyanite have a hardness that depends significantly on direction. Hardness can also be measured on an absolute scale using a sclerometer; compared to the absolute scale, the Mohs scale is nonlinear.
Postage stamp paper
Another comparative set of terms refers to the paper's hardness. "Hard", "stout hard", and "soft" have been used to describe the paper. Experts have described the "snap" of the stamp when "flicked" as a means to determine if the stamp was printed on hard or soft paper. A sharper snap implied hard paper because the hardness is a characteristic of the amount or kind of sizing used when making the paper.