WASHINGTON — There are nine justices on the Supreme Court. It takes four votes to hear a case, but it takes five to stay an execution. That can leave a lethal gap.

A death penalty case can be important enough to claim a spot on the court’s docket of perhaps 75 cases a year. But the prisoner who brought it may not live to see the decision.

In agreeing on Friday to hear a challenge to the chemicals Oklahoma uses to execute condemned prisoners, the court brought fresh attention to the life-or-death importance of a single vote.

The lead petitioner in Friday’s case, Charles F. Warner, was already dead. He was executed eight days earlier, after the Supreme Court refused to stay his execution. The vote was 5 to 4.

“What happened to Charles Warner was not an isolated glitch,” said Eric M. Freedman, a law professor at Hofstra University and the author of a new article on the court’s voting procedures in capital cases. “It was a typical, if high-visibility, example of a systemic flaw in the machinery of justice that has gone unrepaired for far too long.”

The case the court agreed to hear used to be called Warner v. Gross, No. 14-7955. On Friday, taking account of Mr. Warner’s death, the court changed it to Glossip v. Gross, No. 14-7955.

It may change again. The new lead petitioner, Richard Glossip, is scheduled to be executed on Thursday.

The other two petitioners in the case also have execution dates in coming weeks, all of them well before the court is expected to hear arguments in the case, in April.

The Supreme Court did not say on Friday whether it would stay the other three executions. In a statement, Scott Pruitt, Oklahoma’s attorney general, made a pointed reference to the fact that it took only four votes to grant review. He seemed to indicate that the state was prepared to proceed with the executions.

The petitioners’ lawyers will doubtless seek stays. In Mr. Glossip’s case, they will have to act quickly.

How the court responds will illuminate the current vitality of its fitful commitment to a procedure it sometimes uses to bridge the voting gap: the “courtesy fifth” vote to stay executions. Such votes are said to be available once the court makes a formal decision to grant review of a condemned prisoner’s case.

Justice Lewis F. Powell explained his reluctant decision to cast such a courtesy vote in a 1985 concurrence. The inmate’s case had “no merit whatever,” he wrote. “But in view of the unusual situation in which four justices have voted” to hear it, he wrote, “and in view of the fact that this is a capital case with petitioner’s life at stake, and further in view of the fact that the justices are scattered geographically and unable to meet for a conference, I feel obligated to join in granting the application for a stay.”

But such a fifth vote is not always forthcoming. In the late 1980s, the question of when courtesy votes were warranted was a source of tension on the court, and by 1990 the consensus broke down entirely.

“For the first time in recent memory,” Justice William J. Brennan Jr. wrote in a dissent that year, “a man will be executed after the court has decided to hear his claim.”

Thomas C. Goldstein, the publisher of Scotusblog, has written that a courtesy fifth has been uniformly available since the later part of the tenure of Chief Justice William H. Rehnquist, who served from 1986 to 2005. If there are four votes formally to grant review, Mr. Goldstein wrote, a courtesy fifth vote for a stay will always materialize.

But “Supreme Court Practice,” the leading manual on Supreme Court procedure, is agnostic. Mr. Goldstein’s observation “may well be true,” its authors wrote.

“On the other hand,” they added, “no statute or Supreme Court rule requires such a practice, no judicial opinion states that this is the current internal voting procedure of the court, votes on certiorari are generally kept confidential, and the experience of the 1980s reveals that such a practice can break down under pressure.”

The courtesy fifth is, then, a convention rather than a rule, and one that requires an actual vote from an actual justice whose inclinations are necessarily to the contrary. And it seems to apply only after a formal vote to grant review.

In other settings, the gap persists. At least four inmates have been put to death since August over the objections of four justices.

“The court’s willingness to allow executions to go forward with four justices voting for a stay seems coldblooded to me,” said Elizabeth Unger Carlyle, a lawyer for Leon Taylor, one of the inmates.

At the confirmation hearing of Chief Justice John G. Roberts Jr. in 2005, Senator Patrick J. Leahy, Democrat of Vermont, asked him to commit to providing a fifth vote.

“How do you feel if you were chief, if you had four other justices now voting for a stay of execution?” Mr. Leahy asked. “Do you feel, as chief, you should do the courtesy of the rule of five and kick in the fifth one?”

The nominee seemed receptive, if a little tentative.

“I don’t want to commit to pursue a particular practice,” he said. “But it obviously makes great sense.”

In his new article, to be published in The Hofstra Law Review, Professor Freedman considered the history of the court’s practices in this area and made a recommendation. “I propose that in any capital case, regardless of its procedural posture, an execution will be stayed if four justices so desire,” he wrote.

“The justices deserve time to think,” he said. “A statement by four of them that they want that time should suffice to postpone a potentially fatal deadline.”

Whatever it does, he wrote, “the court should address the problem in a reasoned and public way.”