Typographers do sometimes refer to “[old style](https://en.wikipedia.org/wiki/Text_figures)” numbers as being lower case and modern figures as upper case. Lower case digits have varying heights, with the 0, 1 and 2 being small and the other seven digits having ascenders and/or descenders making them taller. In this way the don’t stand out from normal prose on a page in the way more modern digits do due to their being generally larger and a more consistent size.
Quality fonts and typesetting software allows you to choose what kind of digits you want. Often there are four options: old style or modern (“lining”); and proportional or tabular (all digits being of equal width).
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