Incorrect spelling, explanation: -th is a common suffix for ordinal numbers, however not all of them. Some numbers have irregular suffixes, and 1 is one of them – its ordinal form is first, so the suffix in the numeral form will be -st.
21th is, then, a mistake and the correct form is 21st.
Correct spelling, explanation: the -st suffix for ordinal numbers is an irregular suffix for number one. This means that 1 and every other number ending with it (21, 541, 45331, etc. – with an exception for 11 – 11th) will have the suffix -st in the numeral form of its ordinal version. Therefore the correct ordinal form of twenty-one is twenty-first, thus 21st.
21th is a mistake.
Definition of 21st:
numeral, the ordinal number between 20th and 22nd
In the USA 21st birthday is when you can start legally drinking alcohol.
This is a crazy concept to believe in the 21st century!
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