And this what the wikipedia article on stems says:
Handle bar diameter
Both quill and threadless stems come in a variety of
bicycle handlebar clamp diameters. 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 allow for slight differences in handlebar clamp diameter, but the older type of stem with a single pinch bolt must be accurately matched.