Furniture in a formal room is arranged at right angles to each other and the four walls of the room. Pieces are often paired in sets. A sofa might have the same end table on either side of it, and might be placed directly across the room from a similar sofa and end table.
To create a more whimsical feeling in a casual room, arrange the furniture diagonally from each other and the walls, cutting off sharp corners. Don't worry about having sofas, tables or chairs that are too similar, though it always makes sense to have similar or matching colors.