As those of us here in the US already know, US hook sizes are identified by a lettering system. We have our favorites for different yarns, for me H’s, I’s and J’s are great for worsted yarns, G’s and H’s work well for DK, and D’s , E’s and F’s are good for fingering. Whatever our choices, the lettering and sizes are all standard, right? Nope.
Take a look at Ravelry’s hook and needle chart. You can go to your Ravelry library, then click on “needles and hooks”, or you can just look at mine. The hooks are at the bottom, with metric sizes and US letters listed. Notice something funny near the large end of the hook chart? What’s with the M/N, N/P, and P/Q?
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A 9 mm hook is sometimes called an M, and is sometimes called an N. Not only that, but N can be either 9mm or 10mm! Even more dramatic, a P can be 10 mm or 15 mm, or…well, anything inbetween. The numbered sizes are even more confusing – a G hook, for example, is also a US size 6, which is 4 mm (because that makes sense, right?).
Here’s a Boye set, containing sizes L-P. The M is 9mm, N is 10mm, and the P is 11.5mm. They’ve avoided the confusing size numbers entirely. Now check out these hooks from Lion Brand: The Clover Amour onesshow alternate lettering, just like on Ravelry. If you scroll down, though, you’ll see the actual Lion Brand hooks are different: There is no M, but size N-13 is 9mm and size P-15 is 10mm!
Confused yet? Knitting needles are just as bad! What’s listed as a size 1 sock needle could be 2.5mm, or it could be 2.25mm, depending on the brand. There are half sizes, double (or more) 0s, and the larger sizes inexplicably start going in odd numbers only (until you get really large, of course).
So where did this mess come from? Really just a lack of a universal standard. Without any uniform standard, manufacturers created their own, which were all different. Over time, sizes have become more standardized, but as you can see there is still some discrepancy.
The Craft Yarn Council has worked with manufacturers to create a list of sizing standard, and to encourage prominent use of metric measurements. I think metric makes more sense, and the rest of the world is ahead of us in that respect. Why use arbitrary letters and numbers to indicate size, when you can use the actual size instead? After all, it is the size (not the arbitrary label) that really matters.