""" The ISO standard for the clamping area of a handlebar is 25.4 mm (1"), which is used on mountain bikes and many Japanese-made road handlebars. However, the Italian unofficial standard is 26.0 mm, which is the most common clamp size for road bars. There are also intermediate sizes such as 25.8 mm to try and achieve compatibility with either an ISO or Italian stem, and the old Cinelli-specific size of 26.4 mm. In practice, many modern stems with removable faceplates are quite accommodating of slight differences in handlebar clamp size, but the older type of stem with a single pinch bolt must be accurately matched. In the days of quill stems, a road stem was clearly identifiable from its "7" shape, but nowadays it can be hard to tell the difference between a "road" (26.0 mm) and "MTB" (25.4 mm) stem. Manufacturers frequently omit the clamp size from advertising or packaging. A new standard is an "oversize" 31.8 mm or 31.7 mm (1.25") clamp for both MTB and road bars."""

From wikipedia