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Top photo (opposite)
Have the sock's Intended Wearer (IW) clasp the toe of the sock between thumb and hand- grasping as little of the toe as possible.
Bottom photo (opposite) Make a fist, then wrap the sock all way around the hand. You are aiming to get a foot length so that the heel will slightly overlap the toe.
If the heel does slightly overlap the toe, the sock will fit that person.
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This is also a great trick for "trying out" gift socks without revealing what you are making--have the IW shut their eyes (no peeking) and wrap away--if the IW thinks anything, they will think you are knitting them mittens.
A mother's day thought: trying on mystery garments is very intriguing to little kids--they love the drama of "no peeking!" I fondly remember my mom and grandmom doing mystery try-ons for my sister and me--mostly dresses, but sometimes knitted garments--it is one of my favorite childhood memories. Afterwards, my sister and I would speculate on what those garments could possibly be...
--TECHknitter
(You have been reading TECHknitting on: making socks that fit.)