The rule of thirds is a design principle that divides a space or image into nine equal parts by two horizontal and two vertical lines, helping to balance composition by placing elements along these lines or intersections. It relates to the rule of three by guiding the placement of three key objects or color areas to create visual harmony. Combining both helps you check balance across your room or artwork, ensuring focal points align naturally and your trio of decor elements feels anchored and pleasing to the eye.