North, South, and East Capitol Streets and The National Mall divide Washington, DC, into four sections or quadrants: Northwest, Northeast, Southwest, and Southeast. The nexus of the four quadrants is the US Capitol Building. See map below showing the quadrants:

The streets in DC run three ways: east-west, north-south, and diagonally. Lettered streets run east-west, numbered streets run north-south, and diagonal streets have state names.

The National Mall and East Capitol ST run west and east, respectively, away from the US Capitol so all the east-west lettered streets therefore run parallel to them.

Starting at the US Capitol, the first east-west streets north and south of the Capitol are named A ST, the second east-west streets north and south of the Capitol are named B ST, the third C ST, and so on. The street names run through to Y with the letters J, X, and Z skipped. J is skipped because the architect of the DC street system, Pierre L'enfant, thought that the letters I and J looked almost the same typed and when written were indistinguishable from each other. It would have been too confusing to have two street names that looked the same right next to each other. I ST is also referred to as Eye ST.

Once the alphabet to Y is exhausted, the street names are double syllable words from A to Y followed by triple syllable words A to Y followed by the names of trees and flowers. Sometimes, this system is referred to as the first, second, third, and fourth alphabets. See map below showing an example of lettered streets:

North Capitol ST and South Capitol ST run north and south, respectively, away from the US Capitol so all the north-south numbered streets therefore run parallel to them.

Starting at the US Capitol, the first north-south streets east and west of the Capitol are named 1st ST, the second north-south streets east and west of the Capitol are named 2nd ST, and so on. See map below showing an example of numbered streets:

Since there are two of each lettered street and two of each numbered street, the quadrant is appended to the address to define which address is the correct one out of the four possibilities. Take the example below. If a person says they are at K and 12th streets, they need to list the quadrant or else they could be in four different places.

Note that because of this simple grid system, you can always tell exactly where you are in relation to the US Capitol Building. In the example below, if the person is at K and 12th streets NE, then he is 12 blocks east of the Capitol and A, B, C, D, E, F, G, H, I, no J, K, or 10 blocks north of the Capitol. If the person is at K and 12th streets SW, then he is 12 blocks west of the Capitol and 10 blocks south of the Capitol.