THREE SIDE SEAL is a packaging form that is generally square or rectangular in shape with three (3) sealed areas on its sides. The sealed areas are commonly found on the left, right, and bottom sides. That's why many people also refer to Three Side Seal packaging as "SEAL U" (because the sealed areas resemble the letter 'u'). This allows manufacturers to fill the product into the open (top) part. Alternatively, it can also be customized to suit the manufacturer's needs (sealed areas on the left, right, and top). This allows manufacturers to fill the product into the open (bottom) part.