In 1999 the New Times newspaper chain (now part of Village Voice Media, Inc.), owners of ten weekly papers around the United States, approached Hoefler & Frere-Jones for help standardizing production. Ink on newsprint behaves differently in dry Arizona than it does in humid Florida, and page layouts had to be tweaked in each location to appear consistent, which often entailed the redesign of entire pages. To solve this problem, Hoefler & Frere-Jones designed Mercury with a gradation scale, creating letterforms of varying boldness that are nevertheless the same width. (Unlike the weights of a type family, which grow wider as they get bolder, a typefaces's grades increase in darkness without expanding the space the text takes up.) Differences in printing results can be compensated for by adjustment in the grade; because this does not affect the size of the type, page layouts do not have to be redesigned.
from Standard Deviations, 2011