Which architectural element is NOT considered a signature brand identity feature of the architect it's associated with?

In the world of architecture, signature design elements often become synonymous with the brand identity of famous architects. These distinctive features can instantly identify a building's designer without seeing a nameplate. This poll tests your knowledge of how architectural firms have created recognizable brand identities through consistent design languages across their portfolio of buildings.

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Frank Gehry's curved titanium panels
I.M. Pei's perfect square windows
Zaha Hadid's flowing, parametric forms
Santiago Calatrava's white, skeletal structures