Is that really a size 18 dry fly you're tying on?
This gauge tells you hackle size-to-shank length/hook gape accurately after the fly has been tied. Simply grasp the hook bend and pass the hackled portion back and forth through appropriate openings until finding the size that just barely touches or clears the very tips of the hackle collar. Never again fish a fly with an oversized hackle. Whether you're at your tying bench determining proper hook/hackle size, checking hackle size in a fly shop, or verifying the hackle of a fly in your box to match the hatch. Don't be caught fishing a larger fly than belongs on that hook!
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