Pica

Twelve points, and the working unit in which printers measured column widths and page depths.

The pica is the unit printers actually worked in. Points measured type; picas measured everything around it — column widths, page depths, margins, the gutter between columns.

Twelve points make a pica, and a pica is 0.166044 inches. Which means six picas come to 0.996264 inches: just short of one. The temptation to call a pica a sixth of an inch is strong and it is wrong by about four thousandths, a discrepancy that compounds over a page and that compositors simply learned to live with.

Modern desktop publishing resolved it by force. The DTP pica is defined as exactly one sixth of an inch, which makes six picas exactly one inch and makes every pre-1980s measurement very slightly out. The same 0.4 per cent that separates the two points separates the two picas, for the same reason.

The name is thought to come from the medieval pica, a book of church rules, though we have not found that documented in a source we would cite.