PIN number / ISBN number (PIN / ISBN)

Many people add the word "number" to acronyms that already contain it: PIN, ISBN, VIN, SSN, etc. Saying "PIN number" or "ISBN number" repeats "number" and creates redundancy. Keep sentences tighter by using the acronym alone or by spelling it out once for clarity.

Quick answer

Don't add a separate "number" after acronyms whose N already means "number." Say "PIN," "ISBN," "VIN," "SSN," "ATM," etc., not "PIN number," "ISBN number," "VIN number," or "ATM machine."

When the acronym ends with N = number, drop the extra word: "enter your PIN," not "enter your PIN number."
If readers may not know the acronym, spell it out on first use: "Personal Identification Number (PIN)." After that, use the acronym.
In formal writing, always use the concise form.

Core explanation

The redundancy comes from expanding the acronym. PIN stands for Personal Identification Number, and ISBN stands for International Standard Book Number. Adding "number" after either one produces a repetition: "Personal Identification Number number." Removing the redundant word improves clarity and correctness.

This slip often happens because the acronym is familiar in speech, so adding "number" feels natural. In writing, expand the acronym in your head: if the expansion contains "number," don't append a second "number."

Hyphenation and spacing

Hyphenation or spacing errors are related but separate. The "number" redundancy is not about hyphens; it's about meaning. Still, writers confuse spacing in cases like "email" vs "e-mail" or "log in" vs "login."

Check style guides or dictionaries for commonly disputed forms (email/e-mail, login/log in).
For acronyms ending in N, focus on expansion: if the N stands for "number," drop any following "number."

Grammar note

When you encounter an acronym, mentally expand it. If the expansion contains the word "number," you can safely remove any trailing "number" from the sentence. This quick test avoids the repeat without needing a style guide each time.

Real usage: when native speakers slip

People say the redundant forms in everyday speech, and those forms can bleed into informal writing. Here are common contexts and natural rewrites you can use immediately.

Work examples
- Wrong: Please enter your PIN number to access the payroll portal by 5 pm.
  Right: Please enter your PIN to access the payroll portal by 5 pm.
- Wrong: We've recorded the VIN number of the company car.
  Right: We've recorded the VIN of the company car.
- Wrong: Attach the ISBN number when ordering the reference book.
  Right: Attach the ISBN when ordering the reference book.
School examples
- Wrong: Write the ISBN number in your bibliography.
  Right: Write the ISBN in your bibliography.
- Wrong: Do not put your SSN number on the handout.
  Right: Do not put your SSN on the handout.
- Wrong: Enter your PIN number to join the campus Wi-Fi.
  Right: Enter your PIN to join the campus Wi-Fi.
Casual examples
- Wrong: I forgot my PIN number for the phone.
  Right: I forgot my PIN for the phone.
- Wrong: He gave me the VIN number to check the car history.
  Right: He gave me the VIN to check the car history.
- Wrong: Where's the ATM machine near here?
  Right: Where's the ATM near here?

Wrong vs right examples you can copy

Quick pairs to paste into emails, forms, or notes.

Wrong: Please enter your PIN number to access your account.
Right: Please enter your PIN to access your account.
Wrong: The ISBN number of this book is 978-3-16-148410-0.
Right: The ISBN of this book is 978-3-16-148410-0.
Wrong: We logged the VIN number for the leased vehicle.
Right: We logged the VIN for the leased vehicle.
Wrong: The ATM machine is broken.
Right: The ATM is broken.
Wrong: Don't share your SSN number via email.
Right: Don't share your SSN via email.
Wrong: Add the ETA time to the schedule.
Right: Add the ETA to the schedule. (ETA = estimated time of arrival)

How to fix your own sentence

Fixing this mistake is usually quick, but always read the full sentence afterwards to check tone and clarity.

Step 1: Expand the acronym in your head. Does the expansion include "number"?
Step 2: If it does, remove any following "number."
Step 3: Reread the sentence for flow; sometimes a slight rewrite sounds smoother than a direct swap.

Rewrite examples you can reuse:

Original: The shipment's ETA time is unknown.
Rewrite: The shipment's ETA is unknown.
Original: Please enter your PIN number to log in.
Rewrite: Please enter your PIN to log in.
Original: We logged the VIN number for the vehicle.
Rewrite: We logged the VIN for the vehicle.

A simple memory trick

Train yourself to expand acronyms mentally. If "N" expands to "number," do not add another "number." Visualizing the expanded phrase once-Personal Identification Number, International Standard Book Number-makes the correct short form stick.

Search your draft for common offenders (PIN, ISBN, VIN, ATM, ETA, SSN) and fix them in bulk.
When speaking, you can be more forgiving; when writing, prefer the concise form.

Similar mistakes to watch for

Once you make one redundancy error, nearby forms often slip too. Scan for these patterns:

GPS system (Global Positioning System system)
HIV virus (Human Immunodeficiency Virus virus)
LCD display (Liquid Crystal Display display)
ETA time (estimated time of arrival time)

FAQ

Is "PIN number" correct?

No. PIN already contains "number" (Personal Identification Number). Use "PIN."

Do I need to spell out the acronym the first time?

Yes. In formal writing, spell the term on first use with the acronym in parentheses: "Personal Identification Number (PIN)." Use the acronym thereafter.

Can I say the redundant form in speech?

Casual speech often tolerates redundancy, and many people say "PIN number" or "ATM machine" aloud. For written, public-facing, or formal text, use the concise form to appear precise.

What about abbreviations that don't end in N?

Only remove the extra word when the acronym's expansion already contains that word. For acronyms that don't end in N or that don't expand to "number," test each case separately.

How do I stop making this automatically?

When editing, scan acronyms that end in N and expand the N in your head. If it expands to "number," delete any following "number." Consider a search-and-replace for common offenders in long documents.

Want a quick sentence check?

If you're unsure, read the sentence aloud and expand any acronyms mentally. If the expansion contains "number," remove the extra word. A short edit improves clarity and makes your writing look more professional.