That means Mr. Libby’s appeal of his conviction and his remaining sentence can continue. The original sentence was within the range called for by federal sentencing guidelines.

Had Mr. Bush pardoned Mr. Libby, it would have been easier for Mr. Libby to rebuild his life. Mr. Libby’s ability to practice law, for instance, may be affected by the fact that Mr. Bush chose to commute his sentence rather than pardon him.

“The garden-variety pardon is a forgiveness,” said Margaret Colgate Love, the pardon lawyer at the Justice Department for most of the 1990s. “It does not expunge or seal a conviction, but it provides relief from the collateral legal consequences of a conviction.”

Commutations, by contrast, only make the punishment milder. Governors sometimes commute death sentences to life in prison, for instance.

The Constitution gives the president the “power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment.” The provision is generally understood to grant complete discretion where federal crimes are involved.