Merriam-Webster names the everyday redundancy behind ‘ATM machine’ and ‘PIN number’

Such expressions are widespread because they make acronyms easier to recognize.

If you’ve ever said “ATM machine,” “PIN number,” or “LCD display,” you’re far from alone. According to Merriam-Webster Dictionary, those familiar expressions fall under a linguistic phenomenon officially known as RAS syndrome.

In a social media post that quickly caught the attention of language enthusiasts, the dictionary publisher listed several common examples of redundant phrases before revealing the term behind them.

“PIN Number,” “ATM Machine,” “UPC Code,” and “LCD Display,” the post read, before adding: “We regret to inform you that this phenomenon has a name: RAS Syndrome.” The joke continues with the punchline: “Redundant Acronym Syndrome… Syndrome.”

The term describes the habit of repeating a word that’s already represented by the final letter of an acronym.

For example, PIN already means personal identification number, making “PIN number” technically “personal identification number number.” Likewise, ATM stands for automated teller machine, so “ATM machine” expands to “automated teller machine machine.”

Other well-known examples include “HIV virus,” “ISBN number,” “GPS system,” “VIN number,” and “PDF format.”

Despite being technically redundant, linguists note that such expressions are widespread because they make acronyms easier to recognize, especially for people who may not know what the initials stand for. Over time, many acronyms become treated as ordinary words rather than abbreviations, making the extra noun feel natural in everyday speech.

The term “RAS syndrome” itself is intentionally self-referential. Since the “S” in RAS already stands for “syndrome,” saying “RAS syndrome” is itself an example of the very redundancy it describes.

Merriam-Webster’s post sparked amusement online, with many commenters admitting they had unknowingly used these expressions for years—and realizing that even the name of the phenomenon can’t escape its own rule.

“Brought to you by the Department of Redundancy Department,” a commenter wrote.

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