There are quite a few principles of design. They are: balance, emphasis, pattern, repetition, proportion (or size), rhythm, variety, and unity. Emphasis is arguably one of the hardest to make work, since it means creating a focal point in the room without overwhelming the rest of the room. Balance means that the room feels balanced; it’s like a balance beam.
If there’s lots of negative space, or empty space, to one side of the room, there should be about the same on the other side of the room. Unity is the ultimate goal. This means that the room feels united and that every piece flows with the others around it. Well, in interior design. In art in general, these principles help maintain a balance and theory to the piece that everyone loves.