Modular-radial air bearings are available in four basic configurations: The radius may be cut by the length of the bearing or by its width, and either cut may be concave or convex. This offers flexibility, as such bearings can be used on the OD or the ID of a rotating body. They are available for shafts as small as 25 mm and rings as large as 3 m. Larger diameters are accommodated as customs. There are 10 standard-size bearings from 12 × 24 to 150 × 300 mm. These bearings all have a 2:1 aspect ratio, optimal for radial air-bearing functions. Other sizes are available on a custom basis.