XF (Extremely Fine)
A stamp that has all margins clear of the design with all sides of nearly equal width. Although the term is used to describe centering, particularly on U.S. stamps, it also implies that the stamp is sound in every respect. "XF" should not be used for stamps that have minor faults such as light creases, small thins, tiny tears, or faded or dull color and should never be used for stamps with larger faults. The early classics are given slightly more leeway in the description of XF, since well centered stamps are the exception, and the margins need not be huge, but in all cases the margins should be nearly equal in size.