The Council’s hook and needle manufacturers have agreed to make metric (millimeter/mm) sizing more prominent on packaging. The U.S. sizes, both numbers and/or letters, will also appear on packaging.
More prominent labeling of metric sizing, which is an actual measurement, should help eliminate consumer questions about the differences among the letter and number sizing of some needles and hooks. As noted in the introduction, with the cooperation of manufacturers, publishers and designers these changes will be implemented as packaging is reprinted and new patterns are published.
Source: Craft Yarn Council of America’s www.YarnStandards.com