Gum and hinge
In philately, stamp hinges, or mounts, are small, folded, transparent, rectangular pieces of paper coated with a mild gum. They were used by stamp collectors to affix postage stamps onto the pages of a stamp album. Collectors now use different methods to store their stamps. A hinged stamp have a highly reduced value (up to 50%).
NH (Never Hinged)
Never hinged stamps doesn't have any hinge mark on reverse.
Hinge mark on reverse.
LH (Lightly Hinged)
Light hinge mark on reverse.
HH (Heavily Hinged)
HEavy hinge mark on reverse.
HR (Hinge Remnant)
Part of a hinge mark on reverse.
DG (Disturbed Gum)
The gum on reverse is damaged by something else than a hinge (ex.: La gomme au verson a été endommagée par autre chose qu'une charnière (ex.: fingerprint).
NG (No Gum)
Unused stamp wihout gum.
Since the time that unhinged stamps became popular, considerable numbers of old stamps with intact gum have appeared on the market, raising suspicion that many of these have been regummed.
Part of the gum is missing due to an error during the production process.
MNH (Mint or Mint Never Hinged)
The original condition of the stamp.