I recently found a table that explains the difference in crochet hook sizing between the United States sizing and the rest of the world.  Unfortunately, there remains to be a standardized system for crochet and knitting and with the Internet bringing us patterns from all over the world, it can be confusing!  This is true not only for international patterns but for vintage patterns.  With so much of the language of crafting having changed, it is necessary to know exactly what a pattern needs in order to complete it successfully.

Standard Crochet Hook Sizes & Conversion
Smallest to Largest 

For thread, and particularly fine gauge yarn, steel crochet hooks are used.

Steel Crochet Hook Sizes & Conversion
Smallest to Largest

Don’t hesitate to use vintage or foreign patterns.  For a long time, British and European citizens have been forced to convert to American sizing.  Now it’s our turn.  No pattern need be off-limits.  It is just a matter of understanding what sizes convert to the size you are accustomed to using.

Source:  Talking Crochet With Carol Alexander (Conversion Sizes)