Looking at the overhead layout of DC you can see that the Lincoln Memorial and the Capitol lie on a perfect east-west line. The White House and the Jefferson Memorial lie on a perfect north-south line.
Where these lines intersect are the grounds of the Washington Monument. However the monument itself while lying on the east-west line does not touch the north-south line and misses it by (50-100?) yards.
Why didn’t they build it directly at this intersection?
(some googling says something about poor soil conditions but I can’t tell how reliable that is)