For the most expensive clamshell and slate Go stones, stone thickness is carefully controlled and ranked according to size. Typical stone sizes are size 25 (7.0 mm thick) to size 37 (10.4 mm), and even thicker stones are available for a correspondingly much higher price. Commonly used shell and slate Go stone thicknesses are size 32 (8.8 mm) and size 33 (9.2 mm) for most individuals and size 36 (10.1 mm) for professionals. Stones thicker than 10.1 mm are difficult to place on the board, while many Go players prefer stones that are 8.8 and 9.2 mm thick, considering these the optimum thickness for aesthetic and practical reasons.