It’s called a gusset.
Clever Travel Companion Women's Underwear with Hidden Pockets is pretty cool, clever and it works as storage safety. The underwear will definitely
Women's Underwear with Stash Pockets
Why do underwear briefs have pockets? - Quora
Briefs Women Soft Cotton Underwear With Zipper Pocket Solid Comfortable Panties
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Ever wondered why there's a little pocket in your underwear? The answer is not what you think, Lifestyle News - AsiaOne
Women are discovering why there's a small 'pocket' sewn into lining
Panty Gusset 101: Why Do Women's Underwear Have a Pocket?
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What Is The Pocket in Women's Panties For?