18 is as small as I'll tie. Anything smaller I can't fumble finger around with, and can't see them after I cast them. I used to tie down to 24, but those flies now just sit in a box that I never carry anymore. I'm spending more time on stillwater and find that I can usually find something the fish will bite thats tied on a 12 or 14.
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It's wonderful to be good. But it's better if you're lucky and good!