ASTM standards for Screws

That is to say, #4, #6, #8,etc.

The American Society for the Testing and Materials -- an international voluntary group -- has set a uniform standard for the dimension of Wood Screws, Machine Screws and Self-tapping/Sheet Metal Screws, from sizes 2 to 14 shank size. (Larger numbers refer to larger shanks.) Normally, ASTM size is followed by a dash and the number of threads per inch, if threaded. Machine screws have similar markings for sizes, from 000-12, sometimes 14, with a dash, followed by the number of Threads-Per-Inch.

From wikipedia entry on ASTM:

ASTM International is an international voluntary standards organization that develops and produces technical standards for materials, products, systems and services. It was formed in 1898 in the United States as the American Society for Testing and Materials by a group of scientists and engineers, led by Charles Benjamin Dudley, who wanted to address the frequent rail breaks plaguing the fast-growing railroad industry. The group developed a standard for the steel used to fabricate rails.Today, ASTM International maintains more than 12,000 standards....

Sources: Noman F. Harriman Standards and Standardization New York: McGraw-Hill, , 1928; Charles D Sullivan, Standards and Standardization: Basic Principles and Applications New York: Marcel Dekker, 1983.