Ordinal numbers generally express order or rank, rather than a precise quantity. Because they usually address nontechnical aspects of the objects they modify, ordinals are often found in literary writing. The numerical expression of commonly used ordinals (1st, 2nd, 3rd, 4th, etc) may appear jarring and interrupt the flow of the text. For this reason the ordinals first through ninth are spelled out.
The third patient was not available for reevaluation.
It has become second nature.
The numeric form of ordinals greater than ninth is well established in literary texts (10th, 11th, and so on) except at the beginning of a sentence, title, subtitle, or heading. Use the following suffixes: -st, -nd, -rd, -th. These suffixes should not be superscripted.
Eleventh-hour negotiations settled the strike.
The pandemic will continue well into the 21st century.
He celebrated his 80th birthday.
But: Some forms are spelled out by convention, eg, Twenty-fifth Amendment.
If a sentence contains 2 or more ordinals, at least 1 of which is greater than ninth, all should be expressed in numeric form.
Children in the 5th and 10th grades were included in the survey.
The first and third patients treated experienced complete remissions.