Okay, I'll admit that I'm a noob at this but I'm really confused about the carry on size. On Southwest website, it officially says: "Southwest Airlines limits carryon bag dimensions to 10 x 16 x 24 inches." But which measurements are the height/width/length? I was planning on buying a backpack for my carry on and it's 18 height, 12 length, 6 wide. I figured that it would fit within the limits. But not if the carryon has to be less than 10 inches tall! Please help out a newbie!