Correct spelling, explanation: 11th is an ordinal number and is correctly spelled with -th at the end, because the full-word form is pronounced eleventh. Suffix -st is similarly used with numbers that are pronounced with such sound at the end (with word first). This is why 11th is correct and
Definition of 11th:
numeral, ordinal number between 10th and 12th
I will probably take the test late, because I’m 11th on the list.
That’s my 11th tattoo. It’s really addictive!
Incorrect spelling, explanation:
11st is incorrect, because the ordinal number of 11 doesn’t end with first when pronounced. It is spelled with -th, that is why the correct form is 11th (eleventh). The examples of correct usage of suffix -st are 21st (twenty-first), 1001 (two thousand first).
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