A pyrotechnician on July 4 gets a few perks.

There's the freedom to park a truck full of explosives in the middle of the Mall and avoid prosecution. There's the opportunity to combine skills from a trio of disciplines that rarely intersect (floral design, choreography and mortar fire). And there are those great seats for an event that each year prompts hundreds of thousands of people to jockey for position on the Mall.

The nine-man, Tennessee-based crew arranging Sunday's fireworks show has spent months working on a 20-minute display. On Monday evening they arrived in Washington and began unloading six electronic firing boards, unspooling 800,000 feet of electrical detonation wire and loading 2,600 rounds of explosives.

As in recent years, they began lining the Reflecting Pool in front of the Lincoln Memorial with mortar tubes. But this year they have shifted their location just a few yards to the west to make room for the recently opened National World War II Memorial.

"This is the best place to set up in terms of accommodating crowds," said Tom Stiner of Pyro Shows in LaFollette, Tenn., the fireworks company that is coordinating the show for the fourth time. "And that's the important thing -- making room for the crowds."

Those crowds will encounter an event that officials predicted would be similar to those of the past two years, with the same security concerns. Visitors will enter through 19 security checkpoints for casual screenings. In addition to officers from numerous law enforcement agencies patrolling the grounds, the U.S. Park Police and District police will monitor the Mall via security cameras.

Dwight Pettiford, acting chief of the U.S. Park Police, said he thinks people are used to heightened security screening.

"It's kind of like flying -- now when you fly, you know you're going to be screened," he said.

Several events are planned throughout the day on the Fourth, starting at 11:45 a.m. with a two-hour parade on Constitution Avenue between Seventh and 17th streets NW. In the evening, the National Symphony Orchestra will play at 8 p.m. on the West Lawn of the U.S. Capitol. Actor Barry Bostwick will host, and guest performers will include singers Vince Gill, Amy Grant, Clay Aiken and Robin Gibb. The fireworks are scheduled to begin at 9:10 p.m.

National Park Service officials do not estimate crowd sizes, but last year 429,000 riders used the Metro system by 10 p.m. on July 4, compared with 302,000 the year before. Metro plans to operate from 7 a.m. to midnight Sunday.

One place the crowds will not be during the show is the National World War II Memorial, which will be closed to visitors Sunday and Monday. The site will be covered with fire-resistant tarpaulins to protect its surfaces from hot embers and to make cleanup easier for the fireworks crew, officials said.

The fireworks show will be set to a mix of patriotic music, ranging from John Philip Sousa marches to Beyonce's rendition of the national anthem. Head pyrotechnician Scott Goin packaged the show, which means he choreographed the explosions to meld with the music.

"The simple way to explain it is that if they play something like 'Achy Breaky Heart,' then here come the exploding hearts," explained Stiner. "Or if you hear 'Rhapsody in Blue,' boom -- here's the blue fireworks."

"But," Goin said, "it's kind of hard when it's all patriotic music because you can't be doing red, white and blue for all 20 minutes. You have to listen to the music a little more closely."

Stiner said that the Park Service requested a longer finale, so the company has composed a four-minute crescendo this year compared with the slightly shorter ones of years past. Before the finale starts, about one large shell per second will be detonated. Then, during the first two minutes of the finale, about two will detonate per second. For two more minutes, four will detonate per second. In the last 30 seconds, 15 shells will explode per second. Some of those shells include 30 sub-munitions -- individual explosions that make up a single shell.

The shells themselves are 6-, 8- and 10-inch munitions. The industry rule of thumb is that one inch equals 100 feet of lift and 100 feet of airburst diameter, Stiner said. That means the 10-inch shells explode about 1,000 feet in the air and burst into crowns that are 1,000 feet across.

"That's a lot of pops," Stiner said.

The National World War II Memorial, which will be closed to visitors Sunday and Monday, will be draped in fire-resistant tarpaulins before the fireworks extravaganza Sunday night.Pyro Shows workers Chris Irwin and Darby Sparks, bottom left, have been lining the Reflecting Pool with mortar tubes to prepare for the fireworks.