Just for the information, I measured the window sill heights in our house (floor to bottom of sill). They range anywhere from 21 to 42 inches, not including the window over the kitchen sink (46"). I agree with Tom. The higher you install chair rail in a small room, the more overpowering it looks. If you install it too low in a big room, it looks funny. Putting the rail at 21" under the 1950's picture window in the big living room would be really weird but we have a room divider that is 36" high. The height in that room should be 36" to match the divider. So 34 to 36 inches is common but you want to make the chair rail match the room.