You will hardly ever find a room that has absolutely perfectly plumb corners and walls, and level ceilings and floors. And besides, wallpaper stretches when it gets wet with paste, and twists and grows. So it is very common for motifs in wallpaper to appear as if they are moving up or down the wall. In other words, because you cannot count on a ceiling to be level, instead of putting a motif (like the diamond here) at the top of the wall, it’s better to let a little bit of the filler design land at the top. That way, your eye won’t notice if it’s a quarter of an inch from the crown molding, or a half an inch.
In this powder room, despite my best efforts, the diamond eventually worked its way up until it was being cut off by the crown molding. To fool the eye into thinking the ceiling was level, I moved the diamonds down. To do this, I cut the diamonds off of the wallpaper. After the strip of wallpaper was in place on the wall, I replaced the diamonds and pasted them on top of the wallpaper, positioning them so their tips were just below the crown molding.
You can see that the diagonal lines are not lined up perfectly. But this is much less noticeable than if the tops of the diamonds had gotten cut off.
What’s interesting is, I did this for the west end of the room, and part of the south end. But as I made my way around to the final corner of the room, by golly – the crown molding moved back to its original height, and the diamonds moved back down below it, right to where I had plotted for them to be in the first place. :)