Turn Of The Century Valentine "Postcards"

I don't know a lot about old postcards but I do have several dozen very old ones in almost perfect condition. Here are several with a Valentine's day theme. I have them similar to this for Christmas, Halloween, and places of interest. All of them were in a trunk that contained many misc family heirlooms that were put away many many years ago. I'm unsure what it's called when on the backside, these Valentine postcards examples have the indented areas opposite the raised areas on the front. In other words on the side you would write your note, the surface is not smooth but indented which to me, would make it rather difficult to write on, with a pen back in those days when sharp ink pens were used to correspond to others on cards and letters. Anyway these 50 or so "post cards" I have are very unique and it's amazing they are in such great shape still today.