> The flat, broad, flight-enabling feathers we see across most of the wings and much of the body surface of living birds are called pennaceous feathers. (Fun fact: these are the feathers people used to make into quills for writing, hence the word “pen.”)

I get the author's point, I think, but the etymology of "pen" according to wiktionary.com:

> From Middle English penne, from Anglo-Norman penne, from Old French penne, from Latin penna (“feather”), from Proto-Indo-European péth₂r̥ ~ pth₂én- (“feather, wing”), from peth₂- (“to rush, fly”) (from which petition). Proto-Indo-European base also root of *petra-, from which Ancient Greek πτερόν (pterón, “wing”) (whence pterodactyl), Sanskrit पत्रम् (patram, “wing, feather”), Old Church Slavonic перо (pero, “pen”), Old Norse fjǫðr, Old English feðer (Modern English feather);[1] note the /p/ → /f/ Germanic sound change.

So pens aren't called pens because we used pennaceous feathers, but because they were made of feathers, period. At least that's how I get it.

"Pennaceous feather" is a funny term too, then, meaning something like "featherlike feather"?