What kind of flannel shirts don't have the sides that seem partially separated?

I want to know what the type of shirt is called that isn't formal but worn over a t-shirt, yet doesn't seem poofy, nor does it have the separation on the side like lots of shirts that look like it. The kind of shirt that is normally untucked and has ons down the middle, I think its flannel, but whenever I look it up, I always see pictures of the sides with that separation that I don't like. I also only like the slimming version of these shirts, not the poofy ones.