Specially designed wallpaper that allows you to go from one design to the next on the same wall and blending together. My parents have always moved into very old houses that have needed lots of work. One of the things they loved is finding all the different wallpapers from different generations of households. This effect reminds me of that.
The wallpaper is designed by Lene Toni Kjeld, and is available from here. Check out the different patterns and how they merge.