Oldstyle figures in Didot

In old-style figures, the shape and positioning of the numerals varies as those of lowercase letters do. In the most common scheme, 0, 1, and 2 are of x-height, having neither ascenders nor descenders; 6 and 8 have ascenders; and 3, 4, 5, 7, and 9 have descenders. Other schemes exist; for example, the types cut by the Didot family of punchcutters and typographers in France between the late 18th and early 19th centuries typically had an ascending 3 to prevent confusion with the cursive form of the lowercase letter z, a form preserved in some later French typefaces. A few other typefaces used different arrangements.

http://en.wikipedia.org/wiki/Text_figures