Hair-line.—Originally used to indicate the fine line crossing the outer angles of the corner blocks of some British stamps, inserted to distinguish impressions from certain plates, this term is now often employed to denote any fine line, in white or in colour, and whether intentional or accidental, which may be found on a stamp.
Hand-made.—See Paper.
Harrow.—The form of perforating-machine which is capable of operating on an entire sheet of stamps at each descent of the needles. And see Perforation.
Head-plate.—See Key-plate.
Imperforate.—Stamps which have not been perforated or rouletted (both of which see) are thus described.
Imprimatur.—A word usually found in conjunction with "sheet," when it indicates the first impression from a plate endorsed with an official certificate to that effect, and a direction that the plate be used for printing stamps.
Imprint.—The name of the printer, whether below each stamp, or only on the margin of the sheet, is called the "imprint."
Inverted.—Simply upside-down. And see Reversed.
Irregular.—See Perforation.