SW Airlines..How do they measure carryon bags?

I'm flying for the 1st time in 20 years, so I'm not sure how this works. Southwest Air has a 24x16x11 maximum size for carryon bags. I'm packing a duffle bag and it loosely fits within the size restrictions. In other words, the length is about 24", but if someone were to stretch the material, it would probably be as long as 26". Does the airline actually measure the bag or do they do a rough visual check. I am not checking in any bags, so if they restrict my bag, I travel with nothing.SW Airlines..How do they measure carryon bags?
Bags fly free with southwest Airlines.SW Airlines..How do they measure carryon bags?
Most airlines do not actually measure your bags, but if your bag cannot fit into the overhead bin then you would need to recheck your bag and it would cost delay and hassle to both yourself and other passengers. If it's a duffel bag and you don't stuff it to the maximum so it can fit as 24x16x11 it's okay.



BTW Southwest is the only US domestic airline that allows 2 checked bags FREE.