Louis Sullivan

With partner Dankmar Adler, Sullivan pioneered the steel-framed skyscraper as a coherent architectural type — the Wainwright Building (St. Louis, 1891), the Guaranty Building (Buffalo, 1896). His ornamental terra-cotta and cast-iron work fused organic motifs with structural expression.

His credo, "form ever follows function", was misread by mid-century modernists as a stripped-down call to abolish ornament — Sullivan himself never said it implied that, and his work is densely ornamented.

Sullivan's office trained Frank Lloyd Wright, who would carry the Chicago School ethos into the Prairie style and beyond.