On Sun, 26 May 2013 06:17:26 +0000, Danny D wrote:

If anyone knows offhand, why the six ranges - let us know
in a single sentence if it's not a ratio thing.

Ah, digging deeper, I find the six "groups" are numbered
1, 2, 3, 4, 5, 9 (go figure) corresponding to the O-1##,
O-2##, O-3##, etc., designations, for a total of 369
standard sizes.

The first five groups are (mostly) based on cross-sectional
diameter (ranging from 0.040 to 0.275 inch). Within any one of
those cross-sectional groups, inside diameters range from
0.029 to 25.940 inches, counting upward.

Just to confuse you, the sixth group is organized by outside
diameter in 1/16 inch increments.

REF: SAE Aerospace Standard AS568B