While the attacks happened at the Twin Towers in lower Manhattan, the Pentagon across the Potomac River from Washington and a lonely field outside Shanksville, Pennsylvania, the entire nation -- and indeed the world -- absorbed the horror of airliners being turned into weapons.
Demonstrating the enduring legacy of that day in 2001, here are some of the places holding observances around the United States on the 18th anniversary of 9/11:
National September 11 Memorial & Museum
This is the Tribute in Light from 2018. The rousing lights will shine again in 2019.
Spencer Platt/Getty Images
The most shocking images and loss of life came at the World Trade Center complex -- and the site became holy ground for many Americans.
The National September 11 Memorial & Museum opened in 2014 in the same location and today is the main national gathering site documenting the events of 9/11.
Starting at 8:40 a.m. ET Wednesday, the memorial and museum will host its annual commemoration ceremony for family members of the victims of the 2001 attack as well as a 1993 attack.
The memorial and museum will be reopen the public at 3 p.m. Wednesday.
And public is invited to the Tribute in Light that will shine upward from the 9/11 Memorial Plaza, which will be open until midnight. The museum's website says "the lights will be on beginning at sunset on September 11 and will fade away at dawn on September 12."
Elsewhere in New York
St. Paul's Chapel wasn't damaged during the 9/11 attack and was an important rescue staging area in the hours and days afterward. You can see the new One World Trade Center soaring in the background.
Anthony DelMundo/NY Daily News via Getty Images
New York will never forget that day. Here is a small sampling of the many observances being held (all times are local):
-- St. Paul's Chapel of Trinity Church Wall Street: The little Episcopal chapel is directly across the street from the WTC site yet miraculously suffered no damage. At 8:46 a.m., it will ring a Bell of Hope. 209 Broadway, New York, NY 10007; (212) 602-0800 -- Port Authority Remembrance: At 2 p.m., the Port Authority will hold its annual remembrance ceremony to honor its employees who died in the attack. St. Peter's Church, 22 Barclay St, New York, NY 10007; (212) 233-8355
And from 7:15 a.m. to 1:15 p.m., the Port Authority will fly what it says is the world's largest free-flying flag at the George Washington Bridge, part of Interstate 95 that connects upper Manhattan and New Jersey.
Washington, D.C., area
The Pentagon Memorial features 184 empty benches dedicated to the victims of the September 11 attack there.
At the Pentagon, 184 people were killed when hijacked American Airlines Flight 77 was crashed into the building, the potent symbol of US military might.
From 9:30 to 11 a.m. Wednesday, there will be a private ceremony at the Pentagon Memorial for survivors and families of those who were killed. US President Donald Trump is scheduled to attend. The public can visit the memorial after the ceremony is concluded. 1 N Rotary Road, Arlington, VA 22202; +1 301 740 3388
Other observances include:
-- City of Alexandria: At 10 a.m., the Virginia city will hold a remembrance. Market Square, 301 King Street. -- Arlington Cemetery: At 9:30 a.m., people will gather for a brief ceremony and moment of silence. Bozman Plaza flagpoles at 2100 Clarendon Blvd. -- Muslims for Life blood drive: From 8:30 a.m. to 1:30 p.m., the Muslims for Life group works with the Red Cross for a blood drive in honor of 9/11 victimes. Rayburn House Office Building, 45 Independence Ave. SW, 20515; +1 866-236-3276
The visitor Center and viewing platform at the 9/11 memorial for Flight 93 near Shanksville, Pennsylvania.
Thanks to bravery of passengers on Flight 93, the fourth hijacked airline never made it to Washington and instead crashed into a field near Shanksville.
At 9:45 a.m., the annual remembrance will be held at the Flight 93 memorial site. US Vice President Mike Pence is scheduled to speak. 6424 Lincoln Hwy #30, Stoystown, PA 15563; + 1 814 893 6322
Elsewhere in the United States
Showing how far and wide 9/11 reached in distance and time, here is a small list of the many types of ceremonies and observances being held across America:
-- Atlanta: From 9 a.m. to 5:30 p.m., the 9/11 Day of Service organization and Feeding Children Everywhere will join forces to pack meals for hungry kids. College Football Hall of Fame, 250 Marietta St, Atlanta, GA 30313 -- Honolulu: At 12:30 p.m., Chaminade University of Honolulu will hold a never-forget prayer service. Mystical Rose Oratory, 3140 Waialae Ave, Honolulu, HI 96816 -- Memphis, Tennessee: A 9/11 Heroes Run starts at 4 p.m. and finishes up around 9 p.m. Shelby Farms Park, 6903 Great View Drive North, Memphis, TN 38134 -- Milwaukee, Wisconsin: CNN affiliate WTMJ has a podcast of Wisconsin residents who share how their lives were changed by the attacks. -- San Diego: The USS Midway Museum will hold a memorial starting at 2:30 p.m. that should last about two hours. 910 N. Harbor Drive; + 1 619 544 9600 -- Seattle: From noon to 12:30 p.m., the Seattle Fire and Police departments will hold a ceremony. Seattle Center Memorial Garden (south of the International Fountain) 305 Harrison St., Seattle