If you're talking about a dining room, Chair Rails are used to 'cover' this junction. Granted, the Rail's 'width' will 'govern' how much of the joint it will cover. In other words, if you want to use a 2-3/4" wide Rail but your joint varies too much (say, from a level height of 3"), you'll need a rail that's at LEAST 3-1/4" in width. See what I mean?
Hopefully, the 'joint' is either flush or equally 'off-mark'. If it's the latter (which is OK), then you'll need to crown the Rail, that's all. If the joint is 'wavey', it could be tough to make/install a Rail to conceal that imperfection.
My best to ya and hope this helps.
Jay J -Moderator
PS: Your Home Centers and Lumber Yards have TONS of Rails to choose from ...