Square millimeter (mm²)
Definition: A square millimeter (symbol: mm²) is the area of a square with sides of one millimeter. One square millimeter equals one millionth of a square meter.
History/origin: The square millimeter developed as a practical derived metric unit for technical drawing, machining, and small-scale measurement once the metric system became standard.
Current use: Square millimeters are used for wire cross-sections, engineering tolerances, medical measurements, and small product dimensions.
Square kilometer (km²)
Definition: The square kilometer covers a large metric area: one kilometer by one kilometer. Since each side is 1,000 meters, one km² contains 1,000,000 m².
History/origin: Large-scale mapping and public statistics needed a unit bigger than hectares or square meters, so km² became the common metric choice for regions and natural features.
Current use: Km² appears in geography, census tables, environmental reports, park descriptions, city profiles, and comparisons of islands, watersheds, forests, and countries.
