They're very strong, with synthetic woven materials for the outer shell (nylon I think) and metal braces around the top and side corners. The handles are strong too. There is an MDF (eg pressed cardboard) insert on the underside, which provides a solid base, while the inside is lined with a smoother material (the outside is a bit rough). We've used Kirkland K-Cup boxes (they're about the same size) to store things like K-Cups, and these baskets offer the best access. Overall we are very satisfied with this product. P-)
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