Almost all federal judges have at least one law clerk; many have two or more. Associate Justices of the U.S. Supreme Court are allowed four clerks. Although the Chief Justice is allowed to hire five clerks, Chief Justice Rehnquist hired only three per year, and Chief Justice Roberts usually hires only four. Generally, law clerks serve a term of one to two years; however, some federal judges hire a permanent law clerk. Such judges usually have one permanent law clerk and one or two law clerks who serve on a term basis.