Four Quadrants: The address system in D.C. takes some getting used to. The city is divided into the four quadrants of a compass (NW, NE, SE, SW), with the U.S. Capitol at the center. Because the Capitol doesn't sit in the exact center of the city (the Washington Monument does), Northwest is the largest quadrant. Northwest also has most of the important landmarks, although Northeast and Southwest have their fair share.
Numbered Streets and Lettered Streets: Within each quadrant, numbered streets run north to south, and lettered streets run east to west (the letter J was omitted to avoid confusion with the letter I).
The streets form a fairly logical grid—for example, 900 G Street NW is the intersection of 9th and G streets in the northwest quadrant of the city. Likewise, if you count the letters of the alphabet, skipping J, you can get a good sense of the location of an address on a numbered street. For instance, 1600 16th Street NW is close to Q Street, Q being the 16th letter of the alphabet if you skip J.
In short, this means it’s vital to know which quadrant you’re headed to, because 14th Street NW is a long way from 14th Street NE—about 28 blocks, in fact.
Avenues on the Diagonal: As if all this weren't confusing enough, Major Pierre L'Enfant, the Frenchman who originally designed the city, threw in diagonal avenues recalling those of Paris.
Most of D.C.'s avenues are named after U.S. states. You can find addresses on avenues the same way you find those on numbered streets, so 1200 Connecticut Avenue NW is close to M Street, because M is the 12th letter of the alphabet when you skip J. Furthering the chaos, L’Enfant devised a number of traffic circles as well—aesthetically pleasing additions that can nonetheless frustrate even the best out-of-town drivers.