Originally used to describe the fine line crossing the outer angles of the corner blocks of early typographed British stamps (e.g. the “Nine Pence hair-lines”), which were inserted to distinguish impressions from particular plates.

In modern usage, the term covers other fine lines or scratches met with on line-engraved, litho, or photogravure stamps. Where the lines are identifiable as Guide Lines (which see), the latter term is the better.