Personally, I think AICC should be punted over the goal post of so long compliance standards. and some vendors like to offer multiple choices, and yes AICC does different things, but so does driving a moped and I rarely seen people cruising down the interstate on one (unless you live in LA).
Part One: How to Measure Thread Size. 1  In general, the thread should match the fabric in size and weight. For heavier, thicker fabrics, use heavier thread. 2  Heavier threads create more visible stitches. 3  V-69 is the heaviest size recommended for sewing machines. 4  Heavier threads are stronger.
The largest sizes of thread crochet hooks overlap with the smallest sizes of yarn crochet hooks. A demonstration of crochet thread weight: sample filet crochet pattern repeated in different threads. From left to right: size 3, size 10, and size 20. A U.S. quarter is included for perspective. Crochet thread comes in sizes from 3 to 100, although historically it came in much finer sizes, down to 200. Diameter is inversely proportional to number, so size 3 is nearly as thick as yarn and size 100 is as fine as sewing thread.