Correct spelling, explanation: 11th is an ordinal number, and its full, written form is eleventh. 11th – as most of the numbers in English – is a word with a long history as it could already be found in Old English. Back then it had the form endleofan, which could be translated as ten and one left. Eleventh got a similar form to the contemporary one in Middle English.

Definition of 11th:
1. number (ordinal) – comes after tenth and before twelfth,
I was 11th on the entry list to the college, which is quite an achievement.

Phrases and idioms with 11th:
1. at the 11th hour – very late, at the last moment,
We got all the needed information at the 11th hour so it was almost impossible to prepare the project on time.
2. 11th grade – grade at school which comes after the tenth one,
Janet went to the 11th grade with very good results.

Incorrect spelling, explanation: one of the most common mistakes in the spelling of 11th is treating it the same way as 1st. As a result, a common mistake is writing 11th as 11st, which is incorrect. The only ordinal number which takes -st ending is 1.

Incorrect spelling

Incorrect spelling