Q. Do we repeat the abbreviation in ordinal ranges? E.g., should I write “24th–25th” or “24–25th”? In the case of numbers with different ordinal abbreviations it seems the former is obviously preferable (“1–2nd” is obviously infelicitous), but if the two numbers take the same ordinal abbreviation, then it seems (arguably, marginally) better to not repeat unnecessarily.
A. Write ordinal ranges as they would be pronounced; in other words, repeat the ordinal ending in all cases (1st–2nd, 24th–25th, 101st–200th). Another way to look at this is that the ordinal ending is an integral part of an ordinal number and shouldn’t be omitted except in special cases.
One special case is for dates. Days of the month are typically written as cardinals in Chicago (and US) style, though they’re usually pronounced as ordinals: May 9 (pronounced May 9th). A range of days would also be written without ordinal endings: May 5–9. But when a day is used alone or ahead of the month, it’s normally expressed as an ordinal: the 9th of May or, in a range, the 5th–9th of May (where both ordinal endings are required). See also CMOS 9.33.
Note that when you spell out an ordinal, you should write through or to instead of using an en dash: first through tenth (not first–tenth). See also CMOS 6.83 (for en dashes) and 9.6 (for ordinals).