Old
bond issues that have been replaced by a newer issue. When a new Treasury or
federal agency debt issue comes to market, the older issues become off-the-run.
As those issues move further away from their issue date, they become more
difficult to trade because they have less liquidity. Most investors actively
trade only on-the-run issues, which decreases liquidity by lessening interest
in the old, or off-the-run, issues.