Language is full of words that make us pause before we add that final “s” or “es”.
One such word is thorax, a term used in anatomy, zoology, and even entomology.
Its plural form is not only a matter of grammar—it’s also a matter of history and audience.
So, should you say thoraxes or thoraces? The short answer: both are correct, but their usage depends on context.
In this guide, we’ll explore the meaning of thorax, the pluralization rules that apply, why there are two acceptable plurals of thorax, and when to use each form.
By the end, you’ll know exactly which plural form works best for your audience—whether you’re writing for medical professionals or casual readers.
What Is a Thorax?
The thorax is the middle part of an animal’s body that lies between the head and the abdomen. In humans, it’s essentially your chest and back—the section that houses your heart, lungs, ribs, and other vital structures.
In Human Anatomy
- Anatomy term: The thorax includes the rib cage, sternum, and the thoracic section of the spine.
- Protects critical organs such as the heart and lungs.
- Plays a key role in breathing and blood circulation.
Example:
“The surgeon made an incision in the thorax to access the patient’s heart.”
In Insects and Other Arthropods
- The thorax is the central body segment between the head and abdomen.
- It usually contains the legs and wings.
- In crustaceans, it may be fused with the head, forming a cephalothorax.
Example:
“The butterfly’s thorax is covered in fine hairs that help regulate temperature.”
Visual Aid:
| Species | Location of Thorax | Primary Function |
|---|---|---|
| Humans | Between neck and abdomen | Protects heart, lungs; assists breathing |
| Insects | Middle segment | Supports wings and legs |
| Crustaceans | Part of cephalothorax | Supports limbs; aids movement |
The Plural Forms of Thorax
Here’s the part that causes hesitation: What’s the plural of thorax?
- Thoraxes — follows the modern English pluralization rule of adding -es plural to words ending in “x.”
- Thoraces — follows the Latin plural pattern for certain nouns ending in -ax from Greek and Latin origins.
Both are correct. However, thoraxes is more common in everyday English, while thoraces is preferred in technical or medical literature.
Quick Reference Table:
| Plural Form | Type | Usage Context |
|---|---|---|
| Thoraxes | Modern English plural | Newspapers, general writing, casual contexts |
| Thoraces | Classical Latin plural | Medical research, anatomy textbooks, academic papers |
Why Two Plural Forms Exist
The reason for two acceptable plurals of thorax lies in the word’s etymology.
- Originates from the Greek origin “thōrāx” (θώραξ), meaning “breastplate” or “armor.”
- Adopted into Latin as thorax with the plural form thoracēs.
- Entered English through medical and anatomical vocabulary in the 16th century.
English often retains classical plural forms in technical fields, while general speech favors regular pluralization.
Pluralization Rules for Nouns Ending in “-ax”
When deciding on the plural of a noun ending in -ax, you first have to know its origin.
English Rule – Adding -es plural
- Most modern words ending in “x” simply take -es in plural.
- Example: box → boxes, climax → climaxes, thorax → thoraxes.
Latin/Greek Rule – Changing to -aces
- Some scientific and anatomical terms of Greek or Latin origin change -ax to -aces.
- Example: thorax → thoraces, vertex → vertices (note: this is actually -ex → -ices, but shows another irregular Latin plural).
Comparison Table:
| Singular | Modern Plural | Classical Plural | Field of Use |
|---|---|---|---|
| thorax | thoraxes | thoraces | Anatomy, Zoology |
| helix | helixes | helices | Geometry, Biology |
| phalanx | phalanxes | phalanges | Anatomy, Military |
| anthrax | anthraxes | — | Medicine (disease name doesn’t change form) |
Examples in Context
The easiest way to understand plural forms is to see them in use.
Singular: Thorax
- “The insect’s thorax supports three pairs of legs.”
- “A bullet wound to the thorax can be life-threatening.”
Plural: Thoraxes (Modern English)
- “The researchers examined the thoraxes of 20 different beetle species.”
- “X-ray images revealed fractured thoraxes in three patients.”
Plural: Thoraces (Latin/Scientific)
- “CT scans showed fluid accumulation in the thoraces of the study participants.”
- “The textbook includes diagrams of human and insect thoraces for comparison.”
Contextual Usage: Choosing the Right Form
Your choice between thoraxes and thoraces depends on audience familiarity and field of writing.
- General Audience: Use thoraxes. It’s straightforward and matches the English-speaking reader’s expectations.
- Specialized/Scientific Audience: Use thoraces if your readers are accustomed to classical terminology.
Case Study:
A medical journal article from The Lancet used “thoraces” when describing multiple patients’ injuries, while a popular science magazine used “thoraxes” to explain insect anatomy to children.
Both were correct because each publication adapted the plural form to its audience familiarity.
Grammar Rules for Plural Forms
When it comes to grammar rules for plural forms, English offers flexibility—especially for words with classical roots.
- If a noun has an accepted modern plural (thoraxes) and a classical plural (thoraces), both remain correct.
- Maintain consistency: Don’t mix the two plural forms in the same document unless contrasting them.
- For academic credibility in science or medicine, lean toward the Latin plural pattern.
Practical Tips for Writers
- Identify Your Audience
- Technical readers? Use thoraces.
- General readers? Use thoraxes.
- Stay Consistent
- If you start with “thoraces,” don’t switch to “thoraxes” mid-text.
- Use Context Clues
- If you’re talking about human anatomy, the Latin form often feels more formal.
- If you’re writing about insects, “thoraxes” feels more accessible.
Quick Recap Table
| Word | Modern Plural | Classical Plural | Best For |
|---|---|---|---|
| thorax | thoraxes | thoraces | Everyday use vs. scientific contexts |
| helix | helixes | helices | Math, biology |
| appendix | appendixes | appendices | Medicine, publishing |
| phalanx | phalanxes | phalanges | Anatomy, military history |
Final Takeaway
The plural of thorax can be thoraxes or thoraces.
Both are grammatically correct, but your choice depends on audience familiarity and contextual usage.
- Thoraxes → Common in general English writing, easier for non-specialists.
- Thoraces → Preferred in medical, scientific, or academic contexts.
If you remember one thing: match the plural form to your reader’s expectations—that’s how you stay clear, precise, and professional.
If you’d like, I can now create a matching blog image showing a side-by-side diagram of a human thorax and an insect thorax, labeled with both plural forms for visual appeal. That would help with SEO and reader engagement. Would you like me to do that next?