I've been looking at a couple of crochet patterns that are in Japanese, from the Gosyo yarn company website. Their hooks have a different sizing system than US hook sizes. The first chart is a translation of recommended Japanese hooks for different weights of yarn. The second chart translates hook sizing in relation to US sizing.
Translated from Japanese Gosyo yarn company site.
Reference: ABCs of Japanese Knitting. The ABCs charts show translation of Japanese to English for yarn weights, fiber types, numerals, and knitting needle sizes. Very useful. Another useful site is Needleartsbookshop.com, where you find a document for interpreting Japanese Knitting Patterns. Terms like gauge, times, rows, & stitches are interpreted, and there is a list of resources.
|crochet hooks (in Japanese sizes)|
There's a note here that most of the patterns on the Gosyo site use the aluminum hooks sizing.
|US size approx.||Letter||B||B/C||C/D||E||G||H||J||L||N|