Y×XYsubscript𝑋𝑌𝑌{{Y\times_{X}Y}}Y𝑌{{Y}}Y𝑌{{Y}}X𝑋{{X}}π1subscript𝜋1\scriptstyle{\pi_{1}}π2subscript𝜋2\scriptstyle{\pi_{2}}f𝑓\scriptstyle{f}f𝑓\scriptstyle{f}⌟⌟\scriptstyle{\lrcorner}