Base-10 integers, when used as adjectives to express order, add a "st", "nd", "rd", and "th" suffix to whatever the number is, depending on whether the ones digit is a 1, 2, 3, or anything else, respectively. The only exceptions are those ending in 11, 12, and 13, which take "th". Why is there an exception for 11, 12, and 13? You can't tell me that "thirteenth" is less awkward than "thirteenrd"; you're just used to it.
Is this st/nd/rd/th business a unique feature of English? (I know in German, for example, numbers used as adjectives take an adjective ending based on the gender and case of the noun they are modifying. It's not based on rank at all. And in French, you just add "ième" regardless, unless it's the first.)