Introduction
One of the first things you will notice when you tune the low end of any VHF band is that most QSOs include an exchange of "grid squares." What are grid squares? Well, they're more like rectangles and are just a way of dividing up the surface of the Earth. Grid squares are a shorthand means of describing your general location anywhere on the Earth in a manner that is easy to communicate over the air.
An instrument of the Maidenhead Locator System (named after the town outside London where it was first conceived by a meeting of European VHF managers in 1980), a grid square measures 1° latitude by 2° longitude and measures approximately 70 × 100 miles in the continental US. A grid square is indicated by two letters (the field) and two numbers (the square), as in FN31, the grid square within which W1AW, ARRL's Maxim Memorial Station, resides.
Each subsquare is designated by the addition of two letters after the grid square, as FN44ig. These more precise locators are used as part of the exchange in the 10-GHz contest. They measure 2.5 minutes latitude by 5 minutes longitude, roughly corresponding to 3 × 4 miles in the continental US.
Articles
Conversion Between Geodetic and Grid Locator Systems 
QST Jan 1989, pp. 29-30, 43 This simple paper system lets you convert latitudes and longitudes to grid locators.
Web Links
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This calculator by David K2DSL will show your grid square on a map.
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Stephen P. Morse has a web utility for converting addresses to latitude/longitude using a variety of different databases.
- Maidenhead Grid Locator Converter on the AMSAT web page
This one goes to the third two digits! - VUCC Rules Important info if you want to operate precisely on a grid square boundary and have the contacts count for the VUCC award.
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The Summits on the Air Mapping Page offers a dynamic, zoomable Locator grid map and can also be used to look up the grid squares of hilltops used by Amateurs.
