For most of modern history, stocks and bonds were pieces of paper. Sixty years ago, buying a financial security or taking it as collateral meant receiving a certificate about five days later. This worked well enough for decades, but by 1968 there was so much paper that settlement became unmanageable, and the ensuing crisis forced Wall Street to start using computers to keep track of paper securities. This transition took about four years and did not go smoothly. In the time it took for Wall Street to modernize, NASA’s Apollo program managed four moon landings.