Xero Shoes Kelso leather tennis shoes are light and flexible, the upper part is a combination of smooth leather and velor. There is a 3.5mm thick removable insole inside the tennis shoes. The outsole is flexible and has good grip. Upper: Leather Lining: textile Insole: 3.5mm, removable Sole: 6.0mm FeelTrue® Sole Minimalist dimensions for the Xero Kelso model EU Size US Size Inside length Width* 42 9 27 9.5 43 10 27.9 9.7 44 11 28.5 10 45 12 29 10.2 46 13 30.1 10.4 47 14 – – *Width measured from removable insole