From the 1960s to the 1980s, the primary PCB design grid system used Imperial units. All PCB design features and grid layouts were in 0.001" (1 mil) increments and everything was symmetrical and evenly balanced. Then in 1988, the world standards organizations banded together to agree that the metric unit system was superior for solving PCB design development. The first signs of this transition started appearing in the 1990s in component manufacturers' datasheets and the JEDEC component packaging dimensional datasheets, which were once entirely based on Imperial inch units, where slowly converted to metric units.
Metric units appeared in the "PCB Design Grid System." However, this was met by great resistance in the U.S. Some American PCB designers, manufacturing companies, mechanical engineers and electrical engineers are still fighting the transition process.
The transition from one unit system to another introduced chaos into the PCB design industry because PCB designers were forced into using two different unit systems during the transition period. The CAD vendors' way of coping with the transition was to introduce a "gridless shape-based" autorouting feature that provided the PCB designer a solution for working with both metric and imperial unit pin pitched land patterns.http://www.tsingyue.com