A chair rail protects the wall from damage caused by the backs of chairs being pushed against the wall. It takes the form of a horizontal strip of decorative wood, fixed at the same height as the chair backs, so that the wood, rather than the plasterwork, takes the knocks. Chair rails are available in many preformed shapes, or you can create your own from wood moldings nailed and glued together.
Look for the top rail, the obviously wider and usually flat edge. This is the intended upper edge of the rail. Not all rails have a top rail, and many are symmetrical, so this method does not always work.
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Ask a helper to hold a length of chair rail against the wall if there is not an obvious top or bottom to the rail. Ensure that the rail is horizontal and at the desired height above the floor. Rails are usually positioned roughly one-third of the distance from the floor to the ceiling. Step away from the wall and look at the rail. Assess its aesthetics and how well it suits the room. Ask yourself whether it looks good this way up.
Turn the rail around so the top edge is now on the bottom. Step away from it again and decide whether the rail looks better or worse this way round. Select the orientation that looks best since, unless the chair rail has a top rail, there is not definitive top or bottom.
If the chair rail came with packaging, there may be fitting instructions that include diagrams. These may provide clues to the manufacturer's intended orientation for the railing.
There is no right or wrong orientation for a chair rail unless the shape is very obviously intended to have a shelf along the top. It's a design feature that you can fit to express your own likes and room design.