When VisiCalc first (0) APPEARED on personal computers in 1979, few observers grasped the scale of what had just been unleashed. The humble spreadsheet did more than a tedious manual task; it quietly the basic unit of business reasoning. Decisions that had once rested on narrative memoranda were now to cells of numbers whose authority was magnified by their tidy appearance. Over the ensuing decades, the spreadsheet became so to corporate life that its limitations have been widely overlooked. Economists have traced financial crises to modelling errors buried deep in unaudited workbooks, and studies routinely find that a striking proportion of professionally used spreadsheets contain consequential mistakes. The trouble is not the tool itself, which remains extraordinarily powerful, but the confidence it seems to inspire. A well-formatted grid projects a rigour it may not possess, and few users are equipped to the assumptions beneath the formulas. Forty-odd years on, the spreadsheet's greatest lesson may be an uncomfortable one: that the interfaces we trust most are often those we scrutinise