What’s needed for the World Trade Center space, argues Justin Davidson, is something that can serve many functions. A new exhibition gives “a sense of how many simultaneous functions a public space can serve. Italian urbanists long ago understood the beauty of an open square – or ellipse, lopsided trapezoid, or whatever shape streets and houses would permit – on which civic, religious and commercial institutions front and which different generations adapt to their own purposes. These are hybrid areas, where the sacred rubs up against the profane.”