Try this shirt thumbhole design from Threads Digital Ambassador Becky Fulgoni. It is sturdier than in-seam versions and better placed to accommodate each thumb.
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How to Sew Thumbhole Cuffs - Melly Sews
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Customizing With Oliver + S: Thumbhole Sleeve Tutorial, Blog
Thumbhole Cuffs Free Sewing Pattern and Tutorial.
Customizing With Oliver + S: Thumbhole Sleeve Tutorial, Blog
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Sew Sturdier, Better-Placed Thumbholes Into Sleeves - Threads