Words Ending in -x: Indexes or Indices?
Quick Answer
Ordinary English words ending in -x just add -es: one fox, two foxes. Latin words have a second, formal plural in -ices: one index, two indices. Both are correct — they are used in different fields.
See the Pattern First
The everyday words are easy. The Latin ones have two lives.
- One fox, two foxes. Simple — just add -es.
- A book has two indexes at the back.
- A maths paper uses two indices in the equation.
- The river made small vortices — or vortexes. Both are fine.
So the -ices ending is not a harder version of the rule. It is a specialist version, kept alive by maths, science and law.
The Full List
Where two forms are shown, the first is the more common one.
| One | Many | One | Many |
|---|---|---|---|
| fox | foxes | index | indexes / indices |
| appendix | appendices / appendixes | vertex | vertices |
| vortex | vortices / vortexes | codex | codices |
| apex | apexes / apices | matrix | matrices |
These come up often, so they have their own full lesson with more practice: the plural of apex and the plural of matrix.
The Appendix Problem
Appendix is the clearest example of why English keeps both forms — because they came to mean different things:
The extra sections at the back of a book are appendices. The small organ in your body — if we are talking about several people’s — are appendixes. Doctors say appendixes; publishers say appendices. Neither is wrong; they simply work in different worlds.
Everyday: -es. Maths, science, law: -ices.
When -ices Is the Only Option
Three words have effectively lost their -es form. Use -ices:
- matrix → matrices
- vertex → vertices
- codex → codices
These three live almost entirely inside maths, geometry and old manuscripts, so the Latin form never had competition.
Mistakes Learners Make (You Are Not Alone)
Watch for these:
- ✗ two matrixes → ✓ two matrices (matrices is standard in maths)
- ✗ three vertexes of a triangle → ✓ three vertices of a triangle
- ✗ an indices → ✓ an index (indices is already plural)
The last one is the most common by far. “An indices” or “this indices” gives the game away — it must be “an index”.
Your Turn — Quick Quiz
Five questions. Take your time — the table above is still there if you need it.
1. 1. Three ____ ran across the field.
2. 2. In mathematics, the plural of “matrix” is:
3. 3. A triangle has three ____.
4. 4. Which sentence is CORRECT?
5. 5. The book has two ____ at the back.
Keep Going
That is the last of the Latin groups. You can now handle -is, -us, -um, -a and -x with confidence.
Only French is left. Why does one chateau become two chateaux — with an x you do not even pronounce?
Next lesson: French words ending in -eau: bureau, chateau
Sources
I’m Khamis Maiouf. English is not my first language – I learned it as an adult in the UK, passed Functional Skills Level 2 in English, qualified in Hairdressing at Level 3, and I’m now studying Business at university. For over five years I’ve taught ESOL classes in my community and tutored students through GCSE English. I write every lesson on this site myself, for the learner I used to be.
