From Metropolis writer Juliet Whelan: "Some of America’s first urban workers lived in a unique type of Philadelphia home called a Trinity. A Trinity, as the name suggests, consists of three rooms stacked on top of each other. A Betsy Ross stair punches through, basically an elongated spiral stair that is so narrow and steep that, instead of a railing for balance, you haul yourself up using a vertically mounted steel bracket. "Like a typical row house, a Trinity sits on the front property line and comes with a small backyard. Trinities cluster together on quaint little streets, just one cart wide with narrow sidewalks. At first glance, the streets appear to be alleys, but unlike alleys the tiny streets provide front door access to the houses that flank them. Neighbors sit on their stoops and share a drink or play horseshoes together.
"These funny single lane front-streets squeeze in between Philly’s hyper-rational east-west/north-south grid in a willy-nilly fashion, a result of early developers and homesteaders chopping land into blocks as-they-could to claim a piece of dirt for the teeming proletariat. This frenzied speculation resulted in overcrowding and sanitation problems at the time. But now non-through streets function like the suburban cul-de-sac to create a quiet defensible public zone." Full article here.