The most important aspects of stamp condition, to most philatelists, are the presence or absence of faults and the centering. There are other issues to consider, with collecting mint stamps and used stamps, as well.
During the early days of stamp collecting, there were no such things as mounts or hinges. Collectors would moisten the gum on mint stamps and stick them in their albums, other collectors would "glue" used stamps into their albums, and more careful collectors used the gummed sheet margins, folding them to create customized hinges for affixing the stamp into the album space. Of course, these practices resulted in permanently damaged stamps.
Many of the greatest rarities in philately today were originally preserved in this manner. In the early 20th Century, the expert repairing of classical postage stamp was big business. Experts would fill in thin spots by adding liquified paper, adding margins to trimmed imperforate stamps, adding perforations to stamps that were missing some of them, and even adding portions of the stamp designs that had been carelessly trimmed away. Because of the way that early collectors preserved the classics, many of the rarities in the World's finest collections have been repaired in some way. For example, almost all the known examples of the Hawaii Missionary stamps, originally printed on prelure paper, are extensively repaired.
The articles in this section are an overview of the physical properties and historical methods of manufacture that influence how we evaluate condition for both classic and modern postage stamps.
For detailed information on centering and the other determining factors of condition for a particular country or issue, you should consult the specialized catalog for that country. The catalogs usually contain illustrations, to assist in grading and attributing, which I can not use here, due to the fact that most of them are copyrighted material.
Please click on the links at the upper right, to review each article.