Peaking at 555ft (and 5in) and composed of 36,000 blocks of stone, the Washington Monument is the district's tallest structure. Political shenanigans followed by the Civil War interrupted its construction. When work began anew, a new quarry sourced the marble; note the delineation in color where the old and new marble meet about a third of the way up. The monument is currently closed for repairs; check the website for reopening info.
When it reopens, you'll need a ticket to get in. Same-day passes for a timed entrance are available at the kiosk by the monument. During peak season it's a good idea to reserve tickets in advance by phone (877-444-6777) or online (www.recreation.gov) for a small fee.
