So here's a curious thing about that purple crochet hook.
The pattern for the project I'm working on recommends an H / 5 mm hook. The yarn I used recommends a knitting needle size of 5-6 or 3.75-4 mm. Frequently yarn balls give only knitting needle sizes (though this is changing among more enlightened yarn producers). To figure crochet hook size from a recommended knitting needle size, bump up the mm by at least 1.
So, all this suggested that I ought to do my swatch with an H/5mm. I did so. I usually have to go up a hook size to get gauge, so I was rather expecting to be off. But as it turns out, I was just a shade off the gauge, a wee bit too small. If I went up a hook size, it would be too big.
I thought, like Pooh bear, for a moment, then decided to search out my other H hook (I say "other," like I have only one other H hook. I have at least six H hooks. But it was the one I had in mind).
I found the hook, stashed in a project. I checked the size.
Because here's another curious thing: Crochet hooks are marked willy-nilly. The same letter size might have a .5 mm difference. That's a big difference when it comes to getting gauge.
Anyway, the letter and mm sizes matched on the hooks.
The only difference was in the hook itself. The first hook has a sharp lip; the second hook has a round lip. (for an anatomy of a crochet hook, see here.)
I made my second swatch. And the gauge was perfect. I did a little dance in the airport, then I took this picture:
(Ok, so a measuring device would help here, but I worked with the tools I had at the time.) The swatch on top was done with the sharp-lipped hook on top of it, the swatch on the bottom with the round-lipped hook. Look at the right side. See how the bottom swatch is about 1 row wider? It's also longer in the other direction, though that's hard to see. And the fabric on the bottom is happier -- looser with better pattern definition. The round nose made the difference.
Try this yourself if your gauge is just a shade off. Let me know what you learn.
btw, this is the start of a sweater from the Fall issue of Interweave Crochet, out on the stands Oct. 6. Look for a full preview on Friday, Sept 18 and in the next few enewsletters, where we'll also give tips for projects in the issue.