On Wall Street, the older bond, which has about 29 ½ years left to mature, is known as off the run; the shiny new model is on the run. Being less liquid, the off-the-run bond is considered less desirable. It begins to trade at a slight discount (that is, you can purchase it for a little less, or at what amounts to a slightly higher interest yield). As arbitrageurs would say, a spread opens.
