Flatbush Avenue cuts across the Park Slope grid on a diagonal, making it a convenient place to put a subway line and indeed, two separate subway lines employ it, the IRT (2,3,4,5) and the BMT (B, Q) each built in two separate tunnels in the 1910s. It also creates several triangle-shaped plots, one of which can be found at Flatbush, 5th Avenue and Dean Street. For nearly a century, from 1912-2012, the wedge-shaped building in the plot has housed a sporting goods store. By the 1960s, when I began riding by in a bus, the store was called Triangle Sports, quite reasonably enough.
Though the plot sold for over $4M in 2012, the building itself has stood empty since Triangle Sports moved out. This is, of course, the west end of the vast Pacific Park complex (formerly Atlantic Yards), which was spearheaded by the opening of Barclays Center in 2012.
This 1940 photo of the plot, 182 Flatbush Avenue, shows a sporting goods store in place, with ads for Lee clothing, Boy Scout equipment, and firearms. A photographer occupied the second floor. If you look on the right, you can see an elevated train. Until 1940, the 5th Avenue El turned here from Flatbush Avenue; the line originated on the Manhattan end of the Brooklyn Bridge, near City Hall, and its southern terminal was at 3rd Avenue and 65th Street.
When I was a kid, I was in the Cub Scouts. We got my uniform, belts, handbooks etc. from Triangle Sports — but not this one. There was a convenient branch at 5th Avenue and 83rd Street, a block from my house. That particular outlet closed in the 1970s or 1980s.