Correct Usage of Articles (a, an, the) in Scientific Papers

Published June 8, 2024  |  Last updated October 31, 2026

Correct Usage of Articles in Scientific Papers: The Core Rules

English has two articles: the and a/an. Purdue OWL says the is used to refer to specific or particular nouns and a/an to non-specific or non-particular nouns, which it calls the definite and the indefinite article.¹ The table sets out the rules from that page, with short examples written by Editverse for this page.

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Style guides treat missing articles as a problem in their own field too. Google’s developer documentation guide, for example, advises including a, an and the for ease of comprehension and translation, and not skipping them for brevity, including in headings and titles.⁴ It is written for developer documentation, so check your journal’s instructions for authors for title conventions.

A or An: Choose by Sound, Not by Spelling

Merriam-Webster’s usage guide says that in speech and writing a is used before a consonant sound and that an is usual before a vowel sound.² Cambridge Dictionary gives the same rule: an is used instead of a when the following word begins with a vowel sound, as in an orange, an honour and an easy question.⁵ Purdue OWL adds that a goes before words that begin with a consonant sound even when they are spelled with a vowel, such as a user and a university, and that an goes before nouns starting with a silent h, such as an hour.¹

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A or An Before Abbreviations, Numbers and Symbols

The same sound rule applies to abbreviations. Purdue OWL shows initialisms that start with a consonant letter but have a vowel sound taking an: an MSDS and an SPCC plan.¹ Google’s style guide says that pronunciation of abbreviations can vary, so in general you should base the decision on the pronunciation that is most common for your audience, and its word list prefers a SQL, a FHIR and an SAP.³ The rows below are the rules Editverse coded for the analysis that follows, applied to the first sound as read aloud.

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Whichever pronunciation you choose for a term, use it everywhere in the paper. A pair such as an SVM in one paragraph and a SVM in another reads as an error to a reader who says the letters.

The: Specific, Known and Named Things

Use the when both writer and reader know which noun is meant: a noun already introduced, or a noun made specific by a phrase, as in Purdue’s the dog that bit me.¹ Purdue OWL also gives patterns for names of places, shown here with its own examples.¹

What Published Articles Actually Do: Editverse Analysis of 553 CC BY Articles

We checked how published open-access articles choose a or an. Editverse drew a random sample from PubMed Central and tested each pair against the sound rule. The data are our own analysis, not a published study.

Method. On 31 October 2026 we searched PubMed Central through NCBI’s ESearch for records with the CC BY license filter that were e-published on 3 to 7 November 2025, which gave 9,015 unique records, and drew 800 at random (R, set.seed 20261043).⁶ EFetch XML gave the license, article type and body text. All 800 carried a CC BY license URL,⁷ and 553 were English research articles with at least 1,000 body words; they came from 281 journals and held 2,738,856 body words. We read body paragraphs only, leaving out tables, figure captions, formulas, in-text citation markers and reference lists. A pair is a lowercase a or an, preceded by a space and followed by a word or number; we dropped pairs followed by function words such as and, is or in, which are panel labels or variables, and we did not count sentence-initial A and An. That gave 44,693 pairs. A rule coded in R marked each pair consistent or inconsistent with the sound of the next word: words by their first sound (for example hour takes an, unique takes a), numbers by how they are read aloud, and initialisms, symbols and mixed-case terms by the name of their first letter, with a short list of acronyms read as words, and 197 pairs set aside as ambiguous.

Checking the rule. A rule can be wrong, so we read 300 pairs in their sentence, one by one: all 184 pairs the rule flagged, 100 it passed (60 abbreviations, symbols or numbers and 40 ordinary words with a consonant sound spelled with a vowel letter or a silent h) and 16 it had set aside as ambiguous. Each pair was labelled individually as correct, error, not testable or pronunciation-dependent; a brand name or an abbreviation that readers may say as letters or as a word was labelled pronunciation-dependent, never an error. Of the 184 flagged pairs, 125 (67.9%) were confirmed errors, 25 (13.6%) were false flags (brand names, gene symbols and mixed-case initialisms read as words), 23 (12.5%) could be read either way and 11 (6.0%) were not testable because formula text is missing from the XML. All 100 pairs the rule passed were correct (pair-level Wilson 95% CI for the share, 96.3% to 100%), and 13 of the 16 set aside were correct. No article text is published; the per-pair labels are kept by Editverse.

Where the errors are. Confirmed mismatches were rare before ordinary words, 19 of 41,262 pairs (0.05%, 95% CI 0.02% to 0.07%), and before numbers, 6 of 1,655 (0.36%, 0.06% to 0.74%). They were common before initialisms and letter symbols: 100 of 1,776 pairs (5.6%, 4.1% to 7.2%), about 1 in 18. Almost all of those were a before a letter name that starts with a vowel sound; only 2 of the 799 pairs before consonant-sound initialisms used an. Of the 888 pairs before vowel-sound initialisms and symbols, 751 used an and 137 used a, and 98 of those 137 were confirmed errors. Across all articles, 80 of 553 (14.5%, 95% CI 11.8% to 17.6%) contained at least one confirmed mismatch. All of these rates are lower bounds, because only flagged pairs were read in full. The intervals for the three pair rates come from resampling whole articles 2,000 times (R), because mismatches cluster within articles (up to 6 in one article), so they are wider than a pair-level interval would be; the interval for articles is a Wilson interval.

Which letters. Among initialisms and symbols whose first letter name starts with a vowel sound, the share written with an was 100% for E and O and 99.5% for A, but lower for F (65.5%), S (67.4%), N (69.2%), M (72.3%), L (73.9%), R (77.3%) and H (83.1%). The chart covers 871 of the 888 vowel-sound pairs; X and Greek-letter symbols (17 pairs, 16 written with an) are not charted. The letters F, S, N, M, L, R and H are consonants in spelling but begin with a vowel sound in their names. Our counts do not show why writers choose a before them.

Common words. The table gives raw counts of pairs before selected words; rows were counted by the rule and not all were read individually. Terms that can be read as a word or as letters, such as SNP, were set aside from the error counts.

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What this shows. In one week of open-access research articles, the a/an choice before ordinary words almost always followed the sound of the word, while at least 1 in 18 pairs before initialisms and letter symbols did not, mostly a where an was needed. A search for these terms is therefore a worthwhile last check. What it does not show: how closed-access journals behave, whether other weeks differ, how many mismatches the rule missed beyond the 100 passed pairs we read, or why a given pair is wrong. Calling a pair an error depends on reading an initialism letter by letter; we set aside pairs that could be read either way. The text is the one in the PubMed Central XML, which a publisher or conversion step may have changed.

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A Consistency Checklist Before You Submit

1. 1Read every a and an aloud. Say the next word. A vowel sound takes an, a consonant sound takes a, whatever the spelling.
2. 2Settle each abbreviation once. Decide whether your audience says the letters or a word, and use the same article every time.
3. 3Check numbers. Eight, eleven and eighteen take an when read aloud: an 8-week, an 11-year.
4. 4Search the file. Search for an before consonant letters and a before vowel letters, and check each hit in context.
5. 5Check count and noncount nouns. A/an goes only with count nouns; the or no article goes with noncount nouns.
6. 6Follow the journal. Where the instructions for authors name a style guide or dictionary, check the article in that source.

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Frequently Asked Questions

When do I use a and when do I use an in a scientific paper?

Use a before a word that starts with a consonant sound and an before a word that starts with a vowel sound. The choice follows the sound of the next word, not its first letter. Purdue OWL says the choice depends on the sound that begins the next word, and Merriam-Webster says a is used before a consonant sound and an is usual before a vowel sound.

Is it a or an before an abbreviation such as MRI, FDA or SD?

Say the abbreviation aloud. If it is read letter by letter and the first letter name starts with a vowel sound, use an: an MRI, an FDA, an SD. If the first letter name starts with a consonant sound, use a: a PCR, a CT scan. Google developer style guide adds that pronunciation can vary, so base the choice on the pronunciation most common for your audience. In the Editverse sample, 4 of 4 pairs before MRI and 3 of 3 before FDA used an.

Do I use a or an before the word hour?

Use an, because the h in hour is silent and the word begins with a vowel sound. Purdue OWL lists an hour as its example of nouns starting with a silent h. In the Editverse sample, all 9 pairs before hour used an.

Is it a unique or an unique?

Merriam-Webster says a is usual before a consonant sound represented by a vowel letter, as in a one and a union, and notes that an also occurs, although less frequently now than formerly, with an unique as its example. Unique begins with a y sound, so a unique is the standard choice. In the Editverse sample, 76 pairs before unique used a and 1 used an.

Is it a historic or an historic?

Both are used in writing. Merriam-Webster says that before unstressed or weakly stressed syllables with an initial h both a and an are used, and lists a historic and an historic. Purdue OWL says that for historical a is more commonly used and preferred. Pick one form and keep it the same throughout the paper.

Does the a or an rule apply to numbers such as 8, 11 and 18?

Yes, because the rule follows the spoken sound. Eight, eleven and eighteen begin with a vowel sound, so an 8-week course, an 11-year period and an 18-month follow-up take an, while a 6-month follow-up takes a. In the Editverse sample, 68 of 74 pairs before numbers whose spoken form starts with a vowel sound used an and 6 used a.

When should I use the instead of a or an?

Use the when the noun is specific or particular and the reader can tell which one you mean. Purdue OWL says the is used to refer to specific or particular nouns, while a and an refer to non-specific nouns, any member of a group. The can also be used with noncount nouns, or the article can be left out, whereas a and an can be used only with count nouns.

Do rivers, countries and lakes take the?

Purdue OWL gives these patterns. Use the before names of rivers, oceans and seas, such as the Nile and the Pacific, and before deserts, forests, gulfs and peninsulas. Do not use the before most countries, such as Italy, with exceptions such as the Netherlands and the United States, or before cities, single lakes such as Lake Erie, or single mountains such as Mount Everest. A group such as the Great Lakes or a range such as the Andes takes the.

References

1. Purdue University, The Writing Lab and The OWL at Purdue. Using Articles (a/an/the). Purdue OWL. Read 31 October 2026. owl.purdue.edu/owl/general_writing/grammar/using_articles.html
2. Merriam-Webster. an (indefinite article), with the Usage Guide entry “A vs. an”. Merriam-Webster.com Dictionary. Read 31 October 2026. www.merriam-webster.com/dictionary/an
3. Google for Developers. Google developer documentation style guide: Abbreviations, section Indefinite articles before abbreviations (page last updated 1 December 2025; text licensed CC BY 4.0). Read 31 October 2026. A style guide for developer documentation, not a journal rule. developers.google.com/style/abbreviations
4. Google for Developers. Google developer documentation style guide: Articles (a, an, the) (page last updated 21 March 2025; text licensed CC BY 4.0). Read 31 October 2026. A style guide for developer documentation, not a journal rule. developers.google.com/style/articles
5. Cambridge Dictionary. an (determiner). Cambridge University Press & Assessment. Read 31 October 2026. dictionary.cambridge.org/dictionary/english/an
6. National Center for Biotechnology Information. Entrez Programming Utilities Help: ESearch and EFetch (used for the Editverse analysis, queried 31 October 2026). www.ncbi.nlm.nih.gov/books/NBK25499/
7. Creative Commons. Attribution 4.0 International (CC BY 4.0) license summary. creativecommons.org/licenses/by/4.0/

Editverse Research Team | Updated October 2026