What does triplex mean to you? And how do you pronounce the word?
Until last week, it wouldn’t have occurred to me to ask those questions. A triplex, I would have told you with full confidence, is any three-unit residence on a single lot. It could be three units in a single building; a “big house” plus a duplex in the back; or three side-by-side units of equal or varying sizes.

My Platonic ideal of a triplex, in the Los Angeles neighborhood where I grew up (Miracle Mile). Big main unit upstairs, two smaller units downstairs.
And of course it’s pronounced TRY-plex, from Latin roots meaning “threefold.” The first syllable, I was certain, is the same tri- as in triangle, trinomial, trilateral, tripod, trimester, triathlon, tricycle, trisomy, triad, trisect, and Triceratops. The same long vowel as the du- in duplex, the two-unit variation.
I am not, it turns out, entirely wrong. But in many parts of the US and Canada, I am far from right.