That's a linen postcard. It has to do with the linen like finish that the paper has. They still make linen finish paper today. They don't make cards like that today. I really came to like these old cards. I have a nice couple of binders full of sheet protectors with the ones that I saved. I bought a bunch at a local postcard and stamp shop. I don't go there very much anymore. I like cards with interesting old buildings on them. Here's another one from Milwaukee and a plain one from Kansas.