"You have been taught how to duel. Harry Potter?"
-- Voldemort (GF34)
Duelling exists as a sport in the same way that fencing is a sport in the Muggle world. While it is a friendly sporting competition, it is essentially a form of combat. Wizard duelling has rules and competitions, but in a battle, the same skills come into play in life or death struggles.
Dumbledore allowed Lockhart to start a Duelling Club at Hogwarts during the 1992-1993 school year, perhaps in part because he saw trouble coming in the wizarding world and therefore the need for training students in the art of battle. Lockhart taught the formalities of duelling: bowing, holding the wand in a certain way, and flourishing the wand in the attack. Harry found himself, two years later, facing Voldemort in a desperate duel to the death. Voldemort used the Imperius Curse to force Harry to bow and obey the other niceties of duelling. Harry, who had learned to resist the Imperius Curse, used the moment to duck for cover, although he later successfully defeated Voldemort using the one spell he learned from Snape during Duelling Club – the Disarming Charm, “Expelliarmus” (CS11).
The following year, Harry began teaching duelling techniques as part of his Defence Against the Dark Arts study group, the DA. He was so successful that some of the students he’d taught fought well against experienced Death Eaters in the Battle of the Department of Mysteries. Some of them used those same duelling skills against Malfoy, Crabbe, and Goyle aboard the Hogwarts Express on the way home from Hogwarts (June 1996).
Members of Dumbledore’s Army (the D.A.) proved themselves to be valiant duellists in the subsequent battles of the Wizarding war: the Battle of the Tower, The Battle of the Seven Potters, and the climactic Battle of Hogwarts, where many of them even held their own against seasoned Death Eaters such as Bellatrix Lestrange.
Professor Flitwick is rumored to be a champion duellist. It is unknown if this is true and if it is, whether he is a champion of some particular competition.
The term “Warlock” originally referred to a Wizard who was trained in duelling and martial magic, as well as to those who had performed feats of bravery (TBB/WHH).
During the fall of 1995, the Ministry was trying to get other countries to sign an International Ban on Dueling. Percy in particular was working to get the Transylvanians on board (GF23). It is uncertain if this type of duelling is the same as the sport variety or if it refers to the more lethal “pistols at dawn” variety of duel.
A full-on wizard’s duel is something incredible to behold. The duel between Dumbledore and Grindewald in 1945 is legendary (DH2), and the duel between Dumbledore and Voldemort in the Ministry Atrium at the climax of the Battle of the Department of Mysteries was equally spectacular (OP36).
Many duels involve non-verbal spells for speed, but saying the incantations aloud seems to be preferred if there is time (e.g. Voldemort used verbal spells in his duels with Harry). The Shield Charm is one of the most basic duelling spells (BoS). Other spells typically used in duelling include Stunning Spells, the Impediment Jinx, the Full Body Bind, and the Dancing Legs charm. Harry’s use of the Disarming Spell in dueling may be considered somewhat unusual, but this spell is actually one of the key spells to any duelist’s arsenal (BoS).
Aspen wands are favored by those who aspire to be top duelers. In fact, a secretive 18th century dueling club called itself the Silver Spears because they exclusively used aspen wands. Other good wands for duelling include red oak and rowan. Yew wands are feared in duels for their association with curses and evil magic (Pm).
Winners of Duelling competitions include
Alberta Toothill: Winner of the All-England Wizarding Duelling Competition of 1430. Famously overcame the favorite, Samson Wiblin, with a Blasting Charm (FW).
Elizabeth Smudgling, who won the title of Supreme Dueller in a famous Dartmoor contest in 1379 using a spell of her own invention, the Disarming Charm (BoS).
The spelling varies between editions. The British versions use the term 'duelling' while the US editions use 'dueling."
When Harry duels Voldemort in the graveyard, the entire sequence foreshadows their final duel to the death years later:
'"You have been taught how to duel. Harry Potter?" said Voldemort softly, his red eyes glinting through the darkness.
'At these words Harry remembered, as though from a former life, the dueling club at Hogwarts he had attended briefly two years ago. ... All he had learned there was the Disarming Spell, "Expelliarmus". . . and what use would it be to deprive Voldemort of his wand, even if he could, when he was surrounded by Death Eaters, outnumbered by at least thirty to one?' (GF34)
How interesting that Expeilliarmus would be the the spell he WOULD use to defeat Voldemort six years later.
Note the use of colors in the final duel, red, green, and gold:
“You won’t be killing anyone else tonight,” said Harry as they circled, and stared into each other’s eyes, green into red. (DH36)
Harry's eyes are Slytherin green. Voldemort's eyes are Gryffindor red. Stunning spells are Gryffindor red. Killing curses are Slytherin green. When Harry and Voldemore's spells meet, the result is golden flames. And in the end, Voldemort, now called Tom Riddle again, is simply white.
A red-gold glow burst suddenly across the enchanted sky above them as an edge of dazzling sun appeared over the sill of the nearest window. The light hit both of their faces at the same time, so that Voldemort’s was suddenly a flaming blur. Harry heard the high voice shriek as he too yelled his best hope to the heavens, pointing Draco’s wand:
The bang was like a cannon blast, and the golden flames that erupted between them, at the dead center of the circle they had been treading, marked the point where the spells collided. Harry saw Voldemort’s green jet meet his own spell, saw the Elder Wand fly high, dark against the sunrise, spinning across the enchanted ceiling like the head of Nagini, spinning through the air toward the master it would not kill, who had come to take full possession of it at last. And Harry, with the unerring skill of a Seeker, caught the wand in his free hand as Voldemort fell backward, arms splayed, the slit pupils of the scarlet eyes rolling upward. Tom Riddle hit the floor with a mundane finality, his body feeble and shrunken, the white hands empty, the snakelike face vacant and unknowing. Voldemort was dead, killed by his own rebounding curse, and Harry stood with two wands in his hands, staring down at his enemy’s shell.
No matter how many times I've read that passage, I still get goosebumps.