We often hear about how D.C. is divided: East of the river vs. west. Neighborhoods vs. downtown. National capital vs. independent municipality.

But the city is also divided into two worlds that are much more integrated with each other, yet entirely separate: Interior vs. exterior. That is, the face that buildings expose to the street, and the semi-private world behind them, accessible only through the not-quite-streets that trace through the center of a block.

Alleys occupy a special place in D.C.’s history. Unlike in older cities, where alley dwellings were constructed later as real estate pressures mounted, ours were built in conjunction with middle-class housing in the years of the city’s rapid expansion following the Civil War. They housed thousands of poorer residents in tiny dwellings packed into narrow lots that bore little resemblance to the tidy facades fronting the street. “It is a case of Dr. Jekyl and Mr. Hyde in brick and wood, a dual nature incorporated in a prosaic city square,” read a 1909 account of alley life.

Following the creation of the Alley Dwelling Authority in 1934—the city’s first public housing agency—most of these slums were swept away as new housing was created on the edges of the city’s core. Decades later, remnants of that history have been sanitized and elevated to treasured interior spaces, like Blagden Alley and Naylor Court in Shaw, which now houses the D.C. Office of Public Records.

Today’s alley exploration is an unpredictable enterprise. It’s easy to find entrances into the guts of blocks if you’re looking for them; you just have to take a hard right or left when an opening appears. It’s impossible to know where the twists and turns will lead. Backtracking is inevitable. Emerging on the other side of the block is disorienting—even familiar streets look entirely foreign when entered from a different angle.

A willingness to get lost, however, is worth it—a neighborhood’s alleys tell its secrets. Those in Tenleytown are broad and uniform, with occasional glimpses of backyard luxuries: a trailered sailboat, a giant trampoline. Behind Sherier Place NW in Palisades, the street’s inner hippie nature is showcased as pastoral-feeling backyards blend into each other, fence-free. Those backing the taller buildings in the irregular blocks of Dupont Circle are like forests, enclosing zealously-guarded parking spaces. Then, of course, there are the airless caverns downtown, filled with cigarette butts and grease traps and last night’s trash.

But all of them are the more relaxed expression of this uptight metropolis, the spaces that the City Beautiful forgot and can’t control. From this perspective, even the most manicured neighborhoods are embroiled in home maintenance projects, their refuse spilling into walkways. Rowhouses are ramshackle, with awkward additions and rusting cars. On summer evenings, smells of dinner waft out from barbecues and kitchens, making it difficult not to walk up steep staircases and invite oneself in.

It’s that intimacy that separates D.C.’s alleys from its formal rights-of-way. Running into another human being while walking in an alley, you have no choice but to acknowledge their presence. You might also want to hurry on—their business could be personal.

Riding slowly through an alley behind Wisconsin Avenue NW in Georgetown, I come upon an elegantly-attired man leaning on a railing, smoking a cigarette and gazing at the ground in front of him. It’s a dead end, and I can’t escape before he spots me. “I’m just watching my dog,” he says, smiling, and gesturing at a small terrier.

Not sure what to say, I wish him a good evening and turn to find my way out.

“Happy Easter!” he calls out after me.