Those who object to “PIN number” on the grounds that the N in “PIN” stands for “number” in the phrase “personal identification number” are quite right, but it may be difficult to get people to say anything else. “PIN” was invented to meet the objection that a “password” consisting of nothing but numbers is not a word. Pronouncing each letter of the acronym as “P-I-N” blunts its efficiency. Saying just “PIN” reminds us of another common English word, though few people are likely to think when they are told to “enter PIN” that they should shove a steel pin into the terminal they are operating. In writing, anyway,“PIN” is unambiguous and is better used without the redundant “number.”

The same goes for “VIN number”; “VIN” stands for “Vehicle Identification Number.” And “UPC code” is redundant because “UPC”stands for “Universal Product Code.”

Similarly, “ISBN number” would logically mean “International Standard Book Number number.” It’s fine to say just “ISBN,” and that’s what most professionals in the book trade do.