To qualify as a breakfast bar, a place must do one thing -- serve breakfast -- and do it within a limited time span. In the case of the 20th Street Cafe, that span runs from 6 a.m. until 2:30 p.m. weekdays (7 a.m. to 1:30 p.m. Saturdays), and never a minute longer. But if the owners want to knock off just as the late rush is coming to an end, who are we to complain? At 20th Street has been dishing out working-class food for working-class people since 1946. The breakfasts are generous, the coffee always hot, and the chicken-fried steaks a real treat, but what sets this joint apart from all of the other early-morning contenders is the effect of history on this space. Over the years, everything about 20th Street has been worn in and streamlined toward a beautiful conservation of movement. It's small, so the kitchen is never more than a dozen steps away. The waitresses hang dishrags from the coat tree for ease of access. And there's nothing in the canon of American diner cuisine that this kitchen isn't ready to do -- and do better -- than anyone else in town.