First, you need to go back in time to August 2009 when I visited the Castel Beranger in my post Going Gaga for Guimard. This week, I returned to that neck of the woods to get a closer look at another Guimard apartment building at 10, rue Agar. Now I know that agar is that stuff you put in petri dishes to grow cultures, but it also turns out that Agar was an actor who lived in the quartier some 30 years before Guimard designed the building in 1911. (Actually Agar was the stage name of Marie Léonide Charvin. It's the French version of Hagar, which you may recognize from the Old Testament as Abraham's second wife.
In some respects, this building is less extravagant than you might have imagined. It conforms in many ways to the more traditional Haussmanian structures around it.
But it's the little details that give it away. The swooping lines above the doors:
The graceful ironwork:
And of course, like all great works of art, the master's signature.
If you want to take a look, take line 9 to Michel Ange Auteuil or the 52 bus to Leopold II.