For most people, the thought of beheading causes a wave of revulsion and instinctive touching of the neck.
That's the kind of horror Iraqi hostage-takers know they can provoke by beheading their victims, most recently American contractors Eugene Armstrong and Jack Hensley.
Cutting off a human head "is the ultimate symbol of violation and termination," says Robert Thompson, director of the Center for Popular Culture at Syracuse University.
"Clearly the end goal here is to get the attention of a lot of people in a dramatic way and taking off someone's head is about as dramatic as you can get."
From the killers' point of view, the gruesome string of beheadings has won results. Companies in Jordan, Turkey and other countries have pulled their employees out of Iraq, and many Western media organizations have scaled back their coverage.
Too many beheadings, though, and they could lose their impact.
"If this happens once or happens 100 times, it's still going to be horrifying to most healthy people," Thompson says. "But if it starts happening three times a week they're not going to get the attention as the other ones did."
In recent years, beheading as a terrorist tactic has been closely associated with Islamic extremism. Among the most chilling examples were the 2002 beheading of Wall Street Journal reporter Daniel Pearl in Pakistan and last spring's decapitation of American contractor Nick Berg in Iraq.
The Koran, the Muslim holy book, permits beheading but only in the context of warfare, according to one expert.
"There are a couple of verses that say "cut their heads off' but that is specifically about those who have persecuted you, waged war against you or thrown you out of your house," says Yvonne Haddad, professor of Islamic history at Georgetown University.
"The people who took the hostages justify this as war, but everybody else is revolted by these events, including Muslims."
As punishment for criminal acts, beheading is legal in some Muslim countries, most notably Saudi Arabia. Last year the kingdom publicly beheaded 52 men and one woman for murder, rape, sodomy and drug offenses.
In a rare interview, the chief Saudi executioner told the BBC in 2003 that he keeps his government-issued sword sharp as a razor. "People are amazed how fast it can separate the head from the body," he said.
Done correctly, beheading can be as swift and humane as any other method of execution. The prisoner loses consciousness within a few seconds and dies within a minute from shock and lack of oxygen due to the loss of blood.
According to "The History of Beheading" on a British Internet site, the eyes and mouth can move even after decapitation: "It has been calculated that the human brain has enough oxygen stored for metabolism to persist about seven seconds after the head is cut off."
If the blade is dull or the killer sadistic, beheading can be a slow, excruciating death as the executioner hacks through tough muscles and vertebrae in the neck. It took three blows to sever Mary Queen of Scots' head in 1587, and those who have watched the video of Armstrong's killing say he gasped and screamed.
Regardless of how skillfully the beheading is carried out, blood spurts from the severed veins and arteries. That has sparked speculation on the Internet that Berg was killed before he was beheaded, given that almost no blood is apparent on the video that purportly shows his execution.
Although beheading is legal today only in Saudi Arabia, Iran, Qatar and Yemen, it has been used in many non-Muslim countries throughout the ages. The Greeks, Romans and English considered it a more honorable, less painful form of death than other methods of execution, including crucifixion and burning at the stake.
Beheading was also common in France, where Marie Antoinette and thousands of others died at the guillotine during the French Revolution in the 1700s. France used the guillotine as recently as 1977, four years before abolishing capital punishment, and beheading also endured into the 20th century in Norway, Sweden, Germany and China.
Most mainstream media have refused to show photos or videos of the beheadings in Iraq, but they are available on the Internet. Syracuse's Thompson has looked at some: While "a very small percentage" of people may take sick pleasure in watching others die, he says, there is also an understandable curiosity about such horrors.
"We are repulsed and at the same time we feel that if we can only see it, it will seem less scary or empower us somehow. I think there is a natural human tendency when the opportunity arises to take a trip into the heart of darkness."
Susan Taylor Martin can be contacted at susansptimes.com