Numbers come in two heights.
Lining figures are all the same height, roughly the height of a capital, and stand in a row like a fence. Old-style figures vary: the 1, 2 and 0 sit at x-height, the 6 and 8 rise above it, the 3, 4, 5, 7 and 9 drop below the baseline. Both are correct. The job decides which.
In prose
A sentence containing a date wants old-style figures. In 1974 the shop had four presses reads as a sentence when the figures behave like letters, and as a sentence with a small billboard in it when they do not.
Lining figures are cap height, so a four-digit year carries the visual weight of four capitals in a row, and the eye stops on it.
In tables
A table wants lining figures, and tabular ones: every digit the same width, so the columns align without help. Old-style figures break the horizontal reading of a table, and if they are proportionally spaced the decimal points wander down the column.
The bit that trips people up
A face usually has both sets, and the default is whichever the designer expected to be used more. So you ask for the other set explicitly, in CSS with font-variant-numeric, and then check the face actually contains it. Many do not, and the browser quietly ignores the request.1
Where I break the rule
Prices, and anything a reader might need to copy exactly. Charm loses to accuracy at the point where somebody has to write the number down.
Footnotes
-
The dates and reading times on this site use tabular lining figures; the essays use whatever the text face gives by default. That is a compromise, and now you know where it is. ↩
