Paper bulk and caliper.

Caliper is the thickness of a material measured under specified conditions, quoted in thousandths of an inch, called mils or points, or in millimetres (Etherington and Roberts, caliper). Bulk reads the same property from the other end: of two papers of equal weight per ream, the one that handles thicker is said to bulk the better, and in papermaking the word means the ratio of fibre volume to total sheet volume, air spaces included (bulk). PPI is only caliper converted into a book number, and the conversion is old and public, with a caliper of 8.0 thousandths measured over four sheets giving 1,000 pages per inch and 20.0 giving 400 (bulk equivalents). Shops get caught assuming grammage settles thickness, because two papers can weigh the same and bulk differently, and it is caliper the binder cuts to. On a hardcover that gap walks straight into the case, where boards, wrap and hinge are sized from a spine width calculated on the wrong figure and the cover does not fit the block it was made for.