Correct spelling, explanation: 51st is the shortened version of the number fifty-first. Most numbers end with -th, however, there are three exceptions: first, second and third. Because first is a part of this ordinal number, it should be reflected in the abbreviation, so at the end, we should add -st and not -th. Therefore, the correct version is 51st and not 51th.
Definition of 51st:
numeral: ordinal number between 50th and 52nd;
Our house is 51st building on this street.
This is my 51st pair of shoes and I can’t stop myself from buying more.
Incorrect spelling, explanation: 51th is the incorrect version of the abbreviation of 51st. The ending -th is correct for most of the ordinal numbers instead of: first, second, and third, which are the exceptions. For that reason, as the first is a part of fifty-first, the correct form is 51st and not 51th.
Incorrect spelling
Incorrect spelling
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