It is something that mystifies new stamp collectors, until they realize that any antique item that has a faulty part is worth much less than its fit counterpart. That puzzling thing is the back of mint stamps.
One learns soon enough that if the back of a stamp has any blemishes or disturbance to the gum that the value of that stamp must be reduced dramatically. It seems odd that a stamp that sits in an album, showing only its front should be judged so severely on the condition of its back, but that, as Walter Cronkite used to say in signing off "is the way it is."
But it is not only stamps that must be judged for what the world doesn't see. Covers too, if they are missing their backs, loose a great deal of value. The items themselves, having no backs are called... Are you ready? Fronts. They are the wounded soldiers of postal history collecting. They are also mysterious, but not in a good way, as 99% of the time no one has any idea what happened to the backs. Collectors only acquire them when there is no better example of what they are looking for. That is, you will never hear a cover collector ask a dealer "Have you got any good fronts?" Never.