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I play with a Chinese penhold grip and I've seen that some Japanese penholders have a white square on their backhand. I think it's there so their fingers won't slip, which I sometimes struggle with myself. Does anyone know what it is? I think it's some sort of tape, but I'm not sure.
https://www.dreamstime.com/royalty-free-stock-image-kaii-yoshida-jpn-image14473416
https://www.dreamstime.com/royalty-free-stock-image-kaii-yoshida-jpn-image14473416