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Here are some of the key numbers from the battle of the Somme , which began 100 years ago on July 1 :
7 - days of artillery bombardment of the German lines before the battle started, aiming to cut the barbed wire and destroy trench defences and artillery.
1,500,000 - artillery shells fired by the Allies in that week, to little overall effect.
57,470 - British casualties on the first day.
19,240 - British first-day casualties who died.
60 - percentage of British officers involved on the first day who were killed.
141 - days the battle lasted, from July 1 to November 18.
Somme selfies: long-lost photographs of British soldiers involved in the battle
419, 654 - official number of British dead, missing or wounded
1,300,000 - approximate number of casualties on both sides
6 - miles that British soldiers had advanced by the end of the battle.
Show all 30 1 /30In pictures: First World War In pictures: First World War Victoria station, London 1914: A soldier saying goodbye to a loved one in the rain at Victoria station, London, as he leaves for the front
In pictures: First World War Trafalgar Square, London 1914: In Trafalgar Square, London street urchins dressed as soldiers with paper hats and canes as guns stand to attention watched by a small crowd. Behind them is a notice declaring ' The Need for Fighting Men is Urgent'
In pictures: First World War Marylebone Grammar School, London 1914: Two men conscripted to the British Army undergoing a medical check-up at Marylebone Grammar School, London
In pictures: First World War Victoria station, London 1914: Two soldiers on the concourse at Victoria station, London, about to leave for the front line. They are carrying parcels full of food and other provisions
In pictures: First World War British Army 1914: A group of new recruits in training for service in the British Army during World War I
In pictures: First World War Aisne, France 1914: A lone soldier with a bicycle stands amid the remains of a German motor convoy which lines a country lane after an attack by French field guns in the battle of the Aisne in France
Topical Press Agency/Getty Images
In pictures: First World War Aisne River, France 1914: German sharpshooters move to a position near the front line, during the fighting near the Aisne River
In pictures: First World War German naval zeppelin 1914: The L2, a German naval zeppelin during World War I
In pictures: First World War France 1914: French officers dining in style in a trench near the front line
In pictures: First World War Anzac Cove in the Dardanelles 1915: Troops landing at Anzac Cove in the Dardanelles during the Gallipoli campaign of the First World War
Getty Images
In pictures: First World War London 1915: Soldiers arriving at a station in London to travel home for Christmas
In pictures: First World War German Army 1915: A wounded German soldier
In pictures: First World War British Army 1915: A wounded British soldier is stretchered back to camp past a carnage-strewn trench, during the World War I
In pictures: First World War Brighton Pavilion 1915: Injured Indian soldiers of the British Army at the Brighton Pavilion, converted into a military hospital
In pictures: First World War Fort Vaux, France 1916: A German rifleman beside the corpse of a French soldier in a trench at Fort Vaux, France
In pictures: First World War England 1916: Private F.E Henningham leaves for service in the British Army during World War I
In pictures: First World War England 1916: The British soldier, Drummer Bent, wearing his Victoria Cross
In pictures: First World War Somme, France 1916: Gas-masked men of the British Machine Gun Corps with a Vickers machine gun during the first battle of the Somme
In pictures: First World War British Army 1916: British soldiers sitting around a lamp in their trench
In pictures: First World War Austrian Army 1916: Austrian soldiers in the trenches demonstrating their gas masks
In pictures: First World War German Army 1916: Three German soldiers display rats killed in their trench the previous night
In pictures: First World War German Army 1916: A German officer leads his men through a cloud of phosphene gas set off by themselves for cover, as they run toward the British trenches
In pictures: First World War Austria 1916: A dog finds a wounded soldier lying under a tree in Austria during World War I
In pictures: First World War Royal Air Force 1916: Pilots from the Royal Air Force ready to drop bombs by hand over Germany from their aeroplane, a development as in the first stages of the war planes were thought of only as reconnaissance machines
In pictures: First World War WWI aircraft 1916: A group of World War I aircraft flying in formation
In pictures: First World War French and British troops 1916: French and British troops in a trench on the Western Front during World War I
In pictures: First World War Cross Farm, Shackleton, Surrey 1917: Women war workers, at Cross Farm, Shackleton, Surrey
In pictures: First World War American Army in London 1918: American soldiers sightseeing in London from the top of an open-decked omnibus at the end of WW I
In pictures: First World War American Army 1918: A US Army cinematographer filming a US Nieuport 28 biplane taking off during the summer counter-offensive
In pictures: First World War American Army 1918: An American cinematographer sets up his camera in a water-filled trench
49 - Victoria Crosses awarded for valour during the battle.
150,000 - graves in the area cared for in more than 250 military and 150 civilian cemeteries in the Somme area by the the Commonwealth War Graves Commission.
72,000 - number of British and Commonwealth soldiers who died at the Somme with no known graves and whose names are recorded on the British memorial at Thiepval.
Press Association
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