Another weird bag. Laminated xpac1 fabrics are being more common, but I think cotton ones are pretty interesting/unusual. Also it's kind of fun to have a waterproof bag with a cotton exterior that isn't a heavy waxed canvas.
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Xpac is a very strong but stiff and noisy fabric sailcloth (as in, for boats) fabric. It is sometimes laminated to another fabric, such as Cordura which is the most well known "brand name" type of ballistic nylon used in making backpacks in order to have a different surface appearance/feel.