Seen from today's perspective, starting with Basel I, it included very simple rules regarding the capital required if a bank was to lend money. There was a fairly rough breakdown by loan risk, which meant that loans were weighted according to their riskiness.
The belief that it was possible to estimate the loss profiles of various customer and loan types, and hence to calculate the capital requirements for unexpected losses on loans, led to Basel II. As we all know, it did not stop there. We now have Basel III, and complexity has increased. Fortunately, the capital requirements have also increased. Not only capital has been regulated.