That's it. I was silly enough to not have it on my back! Like it should matter.

It was AER 3 so it was 21.5 x 13 x 9 so 3.5 inches (i.e. barely!) too big BUT since it was half empty it totally fit under the seat. My point is either measure it for everyone -- saw much more bulky stuff than mine -- or leave it to "fit" or "not fit". Mine fit under the seat since it was not full thus effectively less than 18 x 14 x 8 and I have photo evidence of it!!!

Side note: since when you want to be a pain in the a** to people who send like 25K+ on you annually (no, I did not pull DYKWIA but I just question business model of calling this for G1 people). Second, I don't run a business, but threatening your high spenders with inconvenience and "putting on a list" seems pretty crazy to me. I've been loyal to AA despite moving to ATL and now SFO but it really triggered me. The person was so aggressive and sure they are right and they are the one to decide what happens to passengers that for the first time (despite sh*it service, delays, cancellations, dating planes, bad hubs) I actually had a talk with my wife of changing to UA/DL or maybe going AS within OW. Each of us have high lifetime milage with AA so this kind of keeps us captured. Never had it with AS despite always carrying the same duo: Amazon Basics approved size carry on + my AER 3. I fly a lot to SEA so AS would make sense (actually make most of my domestic miles as AA codeshared AS) and I love OW but AA was a nice middle ground for international. I know I'm not in the most valuable customers category -- o/w I'd be CK -- but being just below it and being treated like sh*t when I see people boarding after me with bigger backpacks and carry ons just does not feel right.