Maidenhead is the world location system every ham radio operator uses. A 6-character grid like FM18lw (Annapolis, MD) pins your QTH to within a few kilometers. It is short enough for daily-use logging in contest exchanges and on QSL cards, and long enough not to give away your exact street address.
The grid system was defined at the Maidenhead VHF conference in 1980 to give ham operators a compact, globally-unambiguous coordinate form. It uses modular arithmetic on lat/lng with a fixed 18-letter field alphabet and a nested 10-digit / 24-letter subdivision scheme.
Each level adds one resolution step.
Each field is 20° wide by 10° tall.
square is 2° wide by 1° tall, about 220 km across at the equator.
square. Each is 5' wide by 2.5' tall, about 9 km wide by 5 km tall at temperate latitudes.
Each is 30" wide by 15" tall, about 900 m by 460 m. Rarely written in practice; 6 chars is the working ham resolution.
Because the grid uses degrees of longitude (not great-circle distance), the physical width of a grid cell shrinks as you move toward the poles. A 6-character subsquare in Florida is about 9 km wide. The same subsquare in northern Alaska is closer to 5 km wide. Always a fixed 5 km tall.
you exchange grids as the score multiplier. Every new grid you work counts.
heard callsigns with their grid so you can see your signal's path.
great-circle distance and initial bearing without ever exchanging street addresses.
/grid tool: Maidenhead to lat/lng converter at 2/4/6/8character precision, distance and bearing between two grids, plus the visual hierarchy explainer.
feed into automated propagation reporting.