Length is according to the international system measured in meters which is thus a decimal system.
In watchmaking the unit used for length is the millimeter except for tolerances which are expressed in microns also called micrometers. One thousand microns is one millimeter and for reference a hair diameter is about 50 microns.
Another system used in the United States is the feet and thumbs. It is a system that has the particularity to work in dozen. A foot is therefore the result of twelve inches.