What we call Mint Condition, unused stamps with original gum, is classified by collectors into three categories, as follows:
Mint Never Hinged or MNH - This is the best quality of mint stamp, what the Germans call "postfrish" or "post office fresh". In catalogs, this category is frequently denoted as "**".
for U.S. stamps after 1935 and for foreign stamps after about 1950,
they are only considered collectible in this grade. Older stamps that
are never hinged are scarcer, and demand substantial premiums, sometimes
two or three times the catalog value of the same stamp with a hinge
mark on the back.
Mint Hinged or MH - also described as LH (lightly hinged) and HH (heavily hinged) or HR (hinge remnant). In catalogs, this is denoted as "*". As mounts only came into wide use in the early 1970's, many mint stamps collected in the 20th Century are naturally hinged. This is not necessarily bad, and to many collectors, having a lightly hinged mint stamp is not a problem.
For modern issues,
described above, due to their availability and lower demand, hinged
stamps are heavily discounted, sometimes at up to 70% off the catalog
price. For older stamps, the catalog prices, in most cases, are for
Mint No Gum or MNG - denoted in the foreign catalogs by the symbol "(*)". Many special philatelic printings of stamps in the 20th Century were sold without gum. For various reasons, which will be addressed in the Mint Condition - Considerations section, quite a few classic 19th Century mint stamps and some 20th Century mint stamps exist without the original gum, or have had the gum removed for some reason. This is not to say that they are not beautiful, fresh, mint examples, they just have no gum on the back of them. In many instances, the catalog prices for classic stamps, and some other issues, are for examples without gum.
Be careful, when buying high-priced 19th Century mint postage stamps that are not certified or that are not guaranteed by the seller. In the past, there have been dishonest people that have attempted to chemically remove the cancellations (especially pen cancels) from used stamps, or to re-gum unused stamps with out gum, in order to defraud collectors. The seller may be unaware of this, as well. Most sellers are honest, and if the item is not certified, they will offer a money-back guarantee, if the mint stamp is found to be altered or re-gummed. Use of a strong magnifying glass will sometimes reveal signs of re-gumming between the perforations, and putting a stamp in watermark fluid will sometimes reveal shadows in a stamp, where a cancel that has been removed was located. If you feel unsure about an expensive stamp that you are considering buying, the best practice is NOT TO BUY IT.