I get this question from time to time.
The simple answer is hook manufacturers have different US letter (as in the United States, Europe and other parts of the world go by the metric measurement) designations for some of their hooks.
It’s hard to find out how and when this started, but my theory is that because we’ve never adopted the metric system here in the US, the manufacturers came up with the US letters to help us out back in the day.
Why Can’t I Just Get The Same Letter Hook As The Pattern Calls For?
Because it could affect your gauge.
For example, the 4mm could be labelled as either a US-F or a US-G, and a 10mm is either a US-N or US-P. And the reverse is true, a US-N could be a 9mm or a 10mm hook depending on the manufacturer.
If the pattern only gives the US letter hook size, you could potentially be off by one whole mm, which will greatly change your gauge (check out my video below to see more on gauge and hook size).
What Should I Do If The Pattern Only Lists The US Letter Size For The Hook?
The Clover Soft Touch have both letters printed on their hooks. Try to make gauge and go from there (again, see my video down below if you need help with gauge).
So Which Letter Should I Choose if A Pattern Only Has The mm Size Listed?
And the simple answer for that is: don’t worry about letter size. If you have a pattern calling for a hook that comes in multiple letters, stick with the mm size. That is the actual measurement of the hook head and not just the letter the company that made it decided to name it.
You will be much closer to what the designer used when making gauge when you first try to make gauge yourself.