'Form follows function.'

Louis Sullivan

A detail of the cornicework on the Wainwright Building in Saint Louis

Perhaps the most quoted design maxim ever, this pithy phrase is often used in relation to product design, but it was actually coined - by architect Louis Sullivan in relation to the design of skyscrapers. Sullivan was a pioneer of tall building design (he's been called the 'father of skyscrapers' and the 'father of modernism'), and, though he advocated a function-first approach, his skyscrapers, which include the Prudential Building in Buffalo, New York, were not plain; in fact he was happy to use decorative detail as long is it didn't detract from the building's essential usefulness.