Keeping track of numbers has always been part of what makes us human. So at some point along the way, we created a tool to help us keep count, and then we gave that tool a name. We called it: a calculator. But depending on what era you were born in, and maybe even what country, what constituted a 'calculator' varied widely.Keith Houston wrote about the evolution of the calculator in his latest book, Empire of the Sum The Rise and Reign of the Pocket Calculator. It is exactly the kind of nerdery we like to get up to here at 99% Invisible -- history explained through the lens of an everyday designed object.Empire of the Sum