It's a rail...for a chair...right? This mid-wall molding actually has less to do with chairs and more to do with proportion and scale. Knowing how (and why) to use it can drastically affect the look and feel of your space. Here's the scoop.
A colorful (and patterned) Emily Henderson design featuring a chair rail via HGTV
Although it's pretty widely accepted that a chair rail was, at times, used to protect the wall from chair damage in cramped dining rooms (some say that Shaker chair rails were used to hang up dining chairs in order to sweep the floor. Handy!), letting your chair determine the height of your chair rail on your wall is akin to trying to change your foot in order to fit a specific size of shoe —ie. you might get lucky, but it's probably unwise.
The stylish chair rail in the Bellinter House, Ireland
The real purpose of a chair rail is to add visually pleasing architectural interest to your room so the wall's dimensions are what should guide the placement of the molding. We can ignore lots of fancy math and simply estimate that the rail should hit about 1/3 up the wall. Too high and the room will feel squat and "off," so when in doubt, go lower.
Chair rail and bead board circa 1915, Tampa Florida
Another important detail comes at the end of the rail. See above for an example of the proper way to finish the line. The horizontal chair rail should never interrupt or cross the vertical casing of a door (meant to mirror a classic column).
Not only does a chair rail add interest, it also provides separation to a vast expanse of wall that may otherwise seem overwhelming. Dividing the larger area will help you feel more comfortable mixing up color, pattern or texture. Often, it's just the ticket to encourage experimental decor, so don't be afraid to try a chair rail.