Wikipedia
The Bradford system (also known as the English Worsted Yarn Count System or spinning count or Bradford count) is a way to assess the quality of wool.
To measure the fineness of sheep wool fiber before microscopes and lasers were so used, English wool handlers in the city of Bradford described wool by estimating (with experienced eyes) how many 560-yard hanks of single strand yarn could be made by a good spinner from a pound of "top." (Top is cleaned combed wool with the fibers all parallel) The finer the average diameter of a single wool fiber, the more hanks could be spun. From a pound of "64s," for example, sixty-four such hanks could be made (more than 20 miles!). From the finest wools, more than 80 hanks could be spun; from the strongest, perhaps 36 or fewer. Using ranges denoted by the stronger end (that is “44s” ran up to “46s”) wool lots were classified and prices derived.
The Bradford count may be biased no matter how experienced the rater is; also it relies heavily on number of crimps (regular undulations) per inch, which has a not-very-strong correlation with actual average fiber diameter.
More objective measuring systems are rapidly replacing its use in the international market, though it is still widely used among shepherds and breed associations. The United States Department of Agriculture in 1968 issued official standards (for the U.S.A. only, not applicable worldwide) which assigned ranges of average fiber diameter (AFD) and maximum standard deviation to each of the Bradford counts. For example, wool with average fiber diameter in micrometers from 28.60 to 30.09 was to be called "54s." http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELDEV3062803
In the last ten years, objective measurement of several fiber characteristics has become faster and more available and is likely to replace the Bradford count system in all commercial arenas. The spinning count, with all its lore, will not disappear overnight.
Average fiber diameter, although a salient characteristic of sheep breeds and the major determinant of end-uses for wool, is only one of many factors that determines wool quality. For a good discussion of wool classing see Wool Grading, a 1996 article by Rodney Kott of Montana State University.