The following text was written by Folke Rosvall, SM5AGM, one of the inventors of the system;
Background
In the 1950's there was a need among central European VHF and UHF amateurs for a short way of giving positions in contests, because the scoring was based on the distance, normally 1 point per kilometer, and the so called "QRA Locator" (name changed in 1972 to "QTH Locator") was introduced. The system used two letters to indicate the largest unit, "Square", that was 2 degrees longitude * 1 degree latitude. Without repetitions the system covered the area 0-52 degrees Eastern longitude and 40-66 degrees Northern latitude.
The system became very popular and amateurs started to use it in all types of contacts, not only in contests. t also spread outside the non-repeating area and the same locator could unfortunately be found in many places. North American amateurs also started to show interest in the locator idea.
For these and other reasons, the author proposed at a meeting of European VHF managers in Amsterdam in 1976 that we should start discussing a worldwide locator system that could replace the old one. In 1978, Region 1 of the IARU decided to contact the other regions on this matter and the author started to collect proposals for a new system.
In October 1979 the author proposed a system starting at the principal dateline with 20 * 10 degrees "large units", 2 * 1 degrees "middle units" and 6 * 3 minutes "small units", Two months later the author received a letter from Dr. John Morris (G4ANB) who proposed a system starting at the Greenwich longitude with 20 * 10 degrees "large units", 2 * 1 degrees "middle units" and 5 * 2,5 minutes "small units" without having seen the author's proposal. The system were in all other respects identical.
In April 1980 a meeting of European VHF managers was held in Maidenhead near London, UK, where it was felt that the time had come to try to find the best system out of the more than 20 proposals received so far. It was found that the best possible system was the one proposed by G4ANB, with the modification that the starting point should be shifted to the principal dateline, in accordance with what had been proposed by the author.
In 1982 the "Maidenhead Locator System" was adopted by IARU Region 1 as the new locator system starting January 1, 1985.
Description of the locator system
The Earth's surface is divided into 18 * 18 = 324 "Fields", each one 20 degrees longitude * 10 degrees latitude. Each Field is divided into 10 * 10 = 100 "Squares", each one 2 degrees longitude * 1 degree latitude. Each square is finally divided into 24 * 24 = 576 "Subsquares", each one 5 minutes longitude * 2,5 minutes latitude. The Fields are indicated by two letters AA - RR, the Squares by two digits 00 - 99 and the Subsquares by two letters AA - XX. The first character is the longitude and the second character is the latitude on each level. The numbering is always West to East and North to South. The complete locator is the sum of all 6 characters, for example "FN43MJ". Recommended abbreviation on CW for the word "Locator" is "LOC".
A world map showing the 324 fields can be found in "The Radio Amateur's World (Locator) Atlas", that should normally be available at your national amateur radio society.