Among the features assessed are centering, margins, and gum.
[1] Rare or unusual stamps can be submitted for Philatelic expertisation.
The opinions of experts differ and have evolved over time.
[2] Another key factor in the case of mint stamps is the presence or absence of postage stamp gum and whether the gum has been disturbed.
An apparently used stamp with gum, for example from the former Soviet bloc, may be "cancelled to order" ("CTO") and have little or no value to collectors.