1/3rd of an inch is a BARLEYCORN.
1/12th of an inch is a LINE.
1/72nd of an inch is a POINT.
1/100th of an inch is a GRY.
1/144th of an inch is a SECOND.
1/1000th of an inch is a THOU.
1/1440th of an inch is a TWIP.

Via Haggard Hawks. Pleasingly, you can interconvert most of these here.