Prior to the early 1980s, the bond market was comprised mainly of 'plain vanilla' bonds with simplecashflowstructures, where coupon payments andmaturity were fixedat the outset.
For one thing, it is hard to know how much companies have in fact lengthened the maturity of their debts because the interest-rate swap market allows them to swap those fixedbond payments into cheaper floating debt, a popular strategy in the investment-grade market.