This would be a fairly ordinary building, in what we might perhaps call Renaissance style, except for its curious Art Nouveau ornamentation.
The Art Nouveau style never made much headway in Pittsburgh, but there are a few examples of ornamentation in a style that deserves that name—especially stained glass, which lends itself to the kind of abstraction we associate with Art Nouveau. This window is in a storefront near the Birmingham Bridge.
One of old Pa Pitt’s many regrets is that he did not buy this old church on Rhine Street in Spring Hill, merely to preserve its unique Art Nouveau façade. Behind the façade was a pedestrian frame building clinging to the side of the hill, but the façade itself was not quite like anything else in Pittsburgh. This picture was taken in 1999; the church was demolished some time after 2016, when the abandoned hulk still appears in Google Street View. The stained glass probably still exists somewhere; it was removed before the building was demolished.