What is the Semi –circular rule?

For VFR flight between 3,000 feet and 20,000 feet, the semi-circular rule applies and is provided so that aircraft can maintain a minimum distance of 1,000 feet vertically. From 0000 to 1790 Magnetic - ODD THOUSAND plus 500 feet e.g. 3,500 feet, 5,500 feet, 7,500 feet, until 19,500 feet From 1800 to 3590 Magnetic - EVEN THOUSAND plus 500 feet e.g. 4,500 feet, 6,500 feet, 8,500 feet until 18,500 feet