Writing “21th” instead of “21st” is one of the most common mistakes in English, even among advanced speakers. The issue does not stem from a lack of vocabulary, but from a reflex: the suffix “-th” seems logical since it covers the majority of ordinal numbers. However, the digit 1, in the final position, requires a different ending.
Why spell checkers don’t always catch the error 21th
You may have typed “21th” in an email without your software reacting. The built-in correction tools in messaging and word processing applications do not systematically flag this mistake, especially in informal contexts.
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The reason is technical. Spell checkers often distinguish between regular text and text containing numbers. A sequence like “21th” can be interpreted as a code or a reference, not as a word to be checked. As a result, the error goes unnoticed, repeats, and eventually seems normal.
To know which form to use 21st or 21th, one must return to a simple rule that digital tools will not teach you.
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Rule of ordinal suffixes in English: the last digit decides
In English, the ordinal suffix depends on the last digit of the number. Not on the whole number, not on the context, not on the sentence. Only on the final digit.
Here is the complete logic:
- A number ending in 1 takes the suffix “-st”: 1st, 21st, 31st, 41st, 101st.
- A number ending in 2 takes “-nd”: 2nd, 22nd, 32nd, 42nd, 102nd.
- A number ending in 3 takes “-rd”: 3rd, 23rd, 33rd, 43rd, 103rd.
- All others take “-th”: 4th, 5th, 16th, 28th, 50th, 99th.
This is why “-th” seems universal. It covers the vast majority of cases. The brain generalizes, and the error sets in.
Exceptions for numbers ending in 11, 12, and 13: the trap within the trap
The last digit rule works perfectly, except for three numbers: 11, 12, and 13 keep the suffix -th. We write 11th, 12th, 13th, and not “11st”, “12nd”, or “13rd”.
This exception extends to all their hundreds multiples: 111th, 112th, 113th, 211th, 212th, 213th.
Why does this exception exist? In Old English, these three numbers had distinct oral forms that did not follow the pattern of other units. The language has retained this irregularity. Specifically, remember that the “teens” (11, 12, 13) always take -th, regardless of how many hundreds are in front.
21, 22, and 23 are not “teens”. They follow the normal rule: 21st, 22nd, 23rd.
Writing a date with 21st: difference between British and American formats
One of the contexts where “21st” appears most often is in writing dates. And the confusion worsens because the format changes depending on the country.
In British English, the day precedes the month: 21st June 2025. In American English, the month comes first: June 21st, 2025 (with a comma before the year).
Orally, both traditions use “twenty-first” to pronounce the day. The ordinal form is mandatory in speech, even when the written form sometimes uses the cardinal number alone (“June 21” without a suffix, common in informal American emails).
In a formal document (contract, official letter, academic paper), the ordinal suffix is expected. Writing “21th June” in a CV sent to an English-speaking recruiter signals a grammatical gap that is hard to recover from.
When the suffix disappears in writing
In pure digital formats (21/06/2025 or 06/21/2025), no suffix is necessary. The ordinal suffix is only used when the day is written in numbers next to the month in words.
Memorizing the correct ending without a chart
Summary tables are useful for reference, but they are not enough to instill a reflex. Here is a more direct method.
Say the number out loud in English. “Twenty-first” ends with “first”. The written suffix reproduces the end of the spoken word: first gives -st, second gives -nd, third gives -rd. All others end with -th in speech (fourth, fifth, sixth), so -th in writing.
Test with any number:
- 43 → “forty-third” → the spoken word ends with “third” → 43rd.
- 52 → “fifty-second” → the spoken word ends with “second” → 52nd.
- 67 → “sixty-seventh” → the spoken word ends with “seventh” → 67th.
- 21 → “twenty-first” → the spoken word ends with “first” → 21st.
This oral-written correspondence works without exception, including for 11th, 12th, and 13th (pronounced “eleventh”, “twelfth”, “thirteenth”, which all end with a -th sound).
Beyond the rule, it is often the professional context that makes the mistake costly. A quarterly report dated “21th of March” sent to an English-speaking client raises doubts about the overall rigor of the document. In an email subject line, the mistake is visible even before the message is opened.
The correct form is always 21st, never 21th. The error persists because “-th” covers the majority of ordinals and correction tools do not always flag it. By pronouncing the number out loud, the correct suffix appears effortlessly: “twenty-first” leaves no doubt.