When you see initials forming a word, do you automatically call it an acronym? If so, FYI, you might be mistaken.

Take the example of the following transit systems (of course): BART, MARTA, MARC, SEPTA, METRA, MBTA, LIRR, CTA, TTC, and SFRTA.

Would you be surprised to learn only half are acronyms?

That’s right. Only the first five in the list are acronyms.

What are the latter five? Initialisms.

What’s the distinction? Instead of pronouncing the abbreviation as a word, initialisms are pronounced by its individual letters.

So, whereas you’d pronounce BART like a name (acronym), you’d pronounce MBTA as “Em-Be-Tee-Ay” (initialism).

Now, the next time you see an abbreviation, you’ll be able to tell someone a DYK on the subject, which itself is an initialism.