Wallpaper borders can be good accents for room decor. However, because they attract the eye, they can also draw attention to any irregular features of a room that would otherwise be overlooked. A wallpaper border must always be applied level. It is important to consider this and other issues before choosing where to place your wallpaper border.
At the Ceiling
Determine whether the line of the ceiling is visibly unlevel. If so, a wallpaper border could make this more apparent. There is one exception to this rule. If the background colour of the wall is similar to the border and the same as the ceiling, you can often create the look of a level ceiling even if the border doesn't meet up with the ceiling all around the room. If your ceiling has any sloping angles, be sure to consider how your border print will look when mounted at an angle.
At the Soffit
Uneven dimensions can be even more apparent when a border is applied to a soffit. In this case, also consider the visual line at the bottom of the soffit. If companion wallpaper is installed underneath the border, make sure that the border doesn't leave a small strip of background wallpaper uncovered along the soffit.
As a Chair Rail
To determine where to place a wallpaper border as a chair rail, divide the wall into thirds. The centre of the border is usually placed one-third of the way up from the floor. For example, for standard 8-foot ceilings, centre your border at 32 inches off the floor. For 9-foot ceilings, centre your border 6 inches from the floor. But let your eye be the ultimate guide. If your border looks better to you a fraction of an inch or more off these measurements, go with what looks best.
Around Door and Window Frames
If your border won't work at the ceiling, you might want to run it along your door and/or window frames. In this case, it's important to pick a print that will look good when mounted vertically and/or upside down. Choose an appropriate design and be sure to look at it the direction it will run when affixed to the wall before you paste it up.
With a Companion Print
Wherever you decide to mount your wallpaper border, take any background print into account. Ideally, you don't want the two designs to interfere with each other, so try to mount the border over the "background" of your companion print. It's also important to check the border for transparency. If you can see the background print through the border, use a pigmented wallpaper primer/sealer before applying your border. Otherwise, use a vinyl-to-vinyl adhesive to install the border on top of wallpaper.