Like so many great romantic moments in the arts, it begins with the tolling of a bell. The sound dies. Hushed anticipation. Finally, the soldier makes his entrance — no ordinary recruit, but the relief commander of the 3rd U.S. Infantry, taking part in the changing of the guard ceremony at the Tomb of the Unknowns at Arlington National Cemetery.

You could land an airplane on the flatness of his hat, balance teacups on his shoulders. He has been polished and honed to perfection, a man as monument, symbol and embodiment of order, respect and dignity.

Washington life swirls around him — crowds gather and disperse, jets climb into the clouds, the cemetery’s infernal lawn mowers roar. The weather may bake or freeze him — the high, marble mesa on which the tomb rests, at the top of a hill affording one of the loveliest views of the city, can feel like the hottest spot on Earth, even in May. It can also approach the coldest, as when blizzards covered the city two winters ago, and snow buried the plaza as fast as soldiers could shovel it. They replaced their shiny black shoes with combat boots. But the vigil and guard changes went on around the clock, as they do now and have since 1937.

The world doesn’t matter here, in this outdoor theater where the show always goes on. This guard posting is a marathon of purity, a spectacle of the finest abstraction and strictest minimalism, where precise, unthinking repetition blots out just about everything else.

The commander strides across the plaza, past the sarcophagus containing the remains of service members from World Wars I and II and Korea. (An unknown from the Vietnam War had joined them, but his remains were removed and returned to his family when DNA testing revealed his name.) He takes slow, measured steps, rolling his shoes on their outer edges so there’s no hint of a bounce in his body.

It’s the most luxurious legato. The man is a play of contrasts: loose in the knees, square in the chest, all business in the eyes. You know this even though you can’t actually see his eyes because of his sunglasses, so tightly fused to his skull they must be giving him a migraine. But there’s enough expression in his granite jaw to suggest that those hidden eyes are cold. Still, that delicious walk goes on, 18 steps, 20, 21 . . . clack! It’s brought up short, punctuated by a sharp clap of the heels. The metal plates on the inner edges of the shoes are one of many modifications to the basic dress-blues uniform.

The changing of the guard ceremony is like that, a precise, stop-start ballet performed by three men — commander, relief sentinel and the retiring sentinel— alternating between smooth and sharp, silence and staccato pops. With that same liquid gait, the relief sentinel makes his entrance, brandishing the most beautiful, sparkling M-14 you’ve ever seen.

There is a brief, intense inspection, every action crisp and decisive. The commander snatches the relief sentinel’s weapon, swipes its surfaces with an exaggerated sweep of a white-gloved hand. He scrutinizes the uniform — hat, shoulders, belt, shoes, moving his head with such deliberate isolation that if you watch closely you can see the motor impulse travel through each vertebra of his neck. Inspection completed, the ceremony is all about walking and stillness. The men salute the tomb, they fall into step, the two sentinels switch positions, the outgoing one leaving with the commander as the new one takes up his slow pacing along the black rubber mat in front of the tomb.

Soldiers who have performed the ceremony speak of its beauty, its ennobling and even healing power. But while they are posted on the plaza, they make every effort to dehumanize.

Is it ritual or art, functional or pure show? In art as in ritual, context is everything, and this austere plateau of bone-white marble is one of the most loaded frameworks on American soil.

Rituals, as anthropologists tell us, create and cement loyalty among their participants. The military loves a ritual: morning formations, retreat formations, inspections (cleanliness and hygiene having been elevated to a duty, since disease was once the biggest killer on the battlefield), promotion ceremonies, wreath-layings . . .

“It solidifies your present with your past,” says Michael B. Barrett, a retired brigadier general in the Army Reserve who teaches history at The Citadel in Charleston, S.C. “Whether you’ve stormed the heights at Gettysburg or not, you’re sharing the same ritual as those guys did, and you know that 10 to 20 years down the road, some soldier will have taken your place doing the same thing.”

But the ritual surrounding the Tomb of the Unknowns is special. It is a potent symbol of the ultimate sacrifice, made by those who cannot be returned to their loved ones, who perhaps can never be laid to rest, and who gave not only their life but their name to their country. The poignant paradox is that by losing their identity, they have transcended it. The unknown soldier becomes every soldier.

“In a broader sense,” Barrett says, “that tomb represents everyone who has fallen.”

The tomb’s inscription reads: Here rests in honored glory an American soldier known but to God.

But among the earthly, it is the sentinels of the 3rd U.S. Infantry who know him best.

They walk the plaza 24 hours a day, 365 days a year. The guard change happens every 30 minutes during the cemetery’s open hours in the spring and summer and every two hours after the grounds close.

“Sometimes you mesh better with one set of guys over another,” says Benton Thames, 24, assistant sergeant of the guard. “You can feel the rhythm of each other and feel the cadence of the other guys. When it’s really moving, and the heel clicks are crushing and all the pops are loud, you can tell, by the silences and the sounds, that you’ve done a really good job. You just feel it.”

It’s all in the cadence. The soldiers practice their steps with a metronome, set to 72 beats per minute, the tempo of a slow march. A regal adagio time signature. Being chosen as a tomb guard is a rare honor, and even after training for five to seven months, they practice daily. The soldiers assigned to the duty serve 18-24 months. (There have been three female guards in the past, though there are none now.) After the cemetery closes, groups of them will walk the steps over and over. With perfection as the standard — this is written into the “sentinel’s creed” they learn during training — if they stop a half-inch off their mark and their supervisor sees it, they’ll hear about it.

When the new guard is in place, he is still mindful of his cadence. It governs every step — 21 at a time, the number chosen to echo the honor of a 21-gun salute. Then he turns and faces the tomb for 21 seconds. He swivels to face back down the mat, shifts his weapon to the outside shoulder, waits another 21 seconds, takes another 21 steps. This clean geometry and steady rhythm are traced over and over until he is relieved by another guard change.

“People always ask what we’re thinking,” Thames says. “But we’re not really thinking about anything. We’re counting.”

The guard always carries his weapon away from the tomb — and between him and the crowd. Because as much as the ceremony is a popular tourist stop, drawing thousands of onlookers each year, the guards want to keep you at bay. You may grow weak, faint, falter, get in their way — their vigil will go on.

Human reactions have no role here. After a long, steep hike to the top of the hill where the tomb lies, then a period of waiting and standing on those exposed marble steps of the Memorial Amphitheater, visitors are commonly overcome with fatigue. Nearly every day, the soldiers say, someone in the crowd collapses, especially in warm weather. But the guard doesn’t stop counting; he’ll break neither his stride nor his stillness.

“We have to act like it didn’t happen,” Thames says. He describes a time when a man stumbled on the steps, flipped over the chain railing and landed facedown and unconscious on another step.

“You can’t do anything about it,” Thames says. “You have to maintain ceremonial composure.”

Civilian security guards called an ambulance.

The only time the tomb guard will break composure is if the dignity of the ceremony is disrupted. A tourist taking a photograph recently scrambled onto the terrace after his fumbled water bottle started rolling toward the mat. Spec. Mathew Brisiel was the guard at the time, a broad-chested tank of a man standing nearly six-and-a-half feet tall. He stepped off the mat and . . .

“He scared me,” says Staff Sgt. Matt Coffee, who witnessed the event.

Ask why the guards respond to clumsy onlookers, even impulsive children, so forcefully — with a bellowed “request” for silence and respect, an aggressive posture and a jut-jawed stare to pierce stone — and you get a one-word answer.

Intimidation.

Blunt, direct, no nonsense. This is the Army, after all.

Just about every element of guarding the tomb is taken to extremes. Here, spit and polish is a fixation.

“If you’re obsessive-compulsive, the Army is a great place for you,” jokes Staff Sgt. Vern DuBois. We’re in the sentinels’ living quarters under the steps of the amphitheater, where the sentinels stay during their 24-hour shifts. (When not on duty, they are headquartered at Fort Myer with the rest of the 3rd U.S. Infantry, known as the Old Guard.) This is the backstage area, if you will, where the sentinels dress. But they are their own wardrobe mistresses: They spend their downtime working on their uniforms.

Of the three areas in which these soldiers are trained — the ceremony, maintenance of the uniform and knowledge of the cemetery — caring for the uniform is the hardest to master.

“We modify every piece of equipment we’re given,” says Staff Sgt. Lon Baudoux. “The Army doesn’t issue anything that fits.”

The sentinel’s uniform starts out as Army blues. But once these soldiers get hold of it, they gut the jacket, cutting out the lining, sewing on extra buttons and adding pleats down the armhole seams to make the garment fit. They use Stitch Witch bonding tape to stick down all the pockets on the pants. Each element of the uniform is measured to within one-sixty-fourth of an inch, or the thickness of a thumbnail, measured with a metal mechanical-engineering device called a micrometer.

The sunglasses are bent and shaped to each man’s head. They must fit in perfect parallel to the ground, and the hat must be parallel to the sunglasses. Baudoux shows off the metal hooks inside a hat, to keep it from tipping up in front and looking “like a bus driver’s cap.”

The shoes get fussed over the most. They are not permanently glossy like other military dress shoes. Their shine comes from sanding with 200-grit sandpaper and then layers of Kiwi shoe polish, rubbed in and buffed, rubbed in and buffed, for hours a day, depending on the damage. Heat makes the Kiwi melt. Cold makes it bumpy.

The trouble is, no matter how obsessively you polish, that perfect shine remains elusive.

“You can never finish the shoes,” Thames says with a laugh. “You gotta call it quits eventually.”

And should the polishing get tedious, “You just remember why we’re here,” he says. “You pull yourself together, and get ’em shiny.”

They get haircuts twice a week.

A sentinel strides across the room to the large full-length mirror, his face a mask. He studies himself dispassionately. “I need to get tucked,” he calls out. Another soldier comes up behind him to attach the snug-fitting belt, yank the jacket taut across the back and sharpen the pleats. The sentinel is a perfect hourglass. He makes a quarter turn to inspect himself from the side; his shoes click.

Your eye is drawn to the black-handled bayonet at his hip, its scabbard polished like onyx, its leather trim gleaming with a mirror finish. Baudoux picks up an ordinary-looking knife in a drab green scabbard. This is what the Army gives them. After 150 to 300 hours of detailing — each element taken apart, sanded, spray-painted, polished, layer upon layer — Army-issue becomes a work of art.

What happens to the knife also happens to the man.

“They’re a much more disciplined soldier by time they’re done” with the guard training, Baudoux says. “And much more of a soldier that can be — I don’t want to say left to their own devices, but they’re more independent. They mature quite a bit faster than if they were doing a regular job in the Army. They know what’s right, and they do what’s right.”

Another soldier gives the guard a final once-over. “Okay, everything can come out the door,” he says, punching the lock-release button. “Have a good walk.”

What is it to be unknown in an era in which nearly everything is known?

Do we know too much?

Certainly, the Army wants to know.

You’re on the Metro heading to the cemetery for an 8:45 a.m. rendezvous with two Army staff sergeants. At 8:30 you get a call on your cellphone: “Yes ma’am, we just want to know your status.”

The military always wants to know your status. It knows the status of its members — and beyond that, it knows them on a cellular level, too. It’s unlikely there will ever be another unknown soldier, because the DNA of every member of the military is catalogued. With current technology, “you know where your guys are at all times,” Thames says.

But the 3rd U.S. Infantry’s mission is to guard the tomb until the remains are identified. “Which works out to be probably forever,” Thames says.

Forever. No concept is more suited to this undulating landscape, with its majestic trees, this graveyard known as “the gardens of stone.” The spooky romance of the place touches even the heart of a soldier.

Members of the 3rd U.S. Infantry occasionally run through Arlington Cemetery in the mornings before it opens to the public. They aren’t allowed to call cadence — no call-and-response chanting — and they don’t dare spit.

“That’s hard when you’re running,” DuBois says. “But the motivation to keep us in line is, ‘Your fallen comrades are watching.’ ”

Between 10 a.m. and 2 p.m. is considered prime time, when the most experienced soldiers are posted. But Thames says his favorite time is either early in the morning or late in the evening, when the public is gone, “when it’s just us. It’s quiet and peaceful, and there is a beauty to it.”

“I don’t want to say I’ve made friends with the Unknowns or anything,” he says. He strokes his jaw. “But if I’m having a bad day, I’ll stay here till the cemetery closes and get posted for two hours. I go out there . . . and it makes everything okay.”

Now that he has being doing it for four years, some of the emotional power has worn off. But not all. He recalls a time when several World War II veterans in wheelchairs were watching the ceremony. As Thames walked past them with that stately gait, buttons blazing, uniform pressed to a razor sharpness, behind his sunglasses he could see the old soldiers pushing down on their armrests, trying with all their trembling might to stand.

“Those that could saluted me as I passed,” Thames says, swinging his right hand up to his brow with a shy smile, a gesture both casual and elegant.

“That kind of got to me.”

A veteran once told Thames that he’d lost a buddy in World War II and that the body was never recovered. When he comes to the Tomb of the Unknowns, the veteran imagines that those remains belong to that guy — and this becomes the place where he can be mourned as if his name were cut in stone.

This is why the sentinel buffs his shoes, hollers for a good tucking-in, submits to having his creases measured to within a fraction of an inch. This is why he has seemingly shaved away every shred of his own individuality, his identity, for a task whose purpose is, at the heart of it, exquisitely tender. It is the physical expression of an intangible wish, the fulfillment of a promise.

Long past Memorial Day.

“All soldiers recognize that it represents them,” says Barrett, The Citadel professor. “Underlying the tomb is that if something happens to you and we can’t identify you or find you, that ceremony still honors you.

“We ask them, if necessary, to lay down their lives,” he continues, his voice faltering with emotion, for he was once a soldier. “This is the corollary: They will not be forgotten.”