"The cores reacted in a way that Lord Voldemort, who never knew that your wand was a twin of his, had never expected. He was more afraid than you were that night, Harry."
-- Dumbledore (DH35)
Two wands that have the same magical substance at the core are said to be twins or brother wands.
- The wands of Harry and Voldemort each contained a single phoenix feather given to Mr. Ollivander the wandmaker by Fawkes (PS5, GF36).
- When Voldemort dueled against Harry in the Little Hangleton graveyard, the twin wand cores connected in the rare Reverse-Spell Effect, Priori Incantatem. Voldemort's wand had to "regurgitate" spells in reverse, and since all of them were the Killing Curse, Harry saw spirits of the dead, including Cedric Diggory, Frank Bryce, Bertha Jorkins and his own parents, James and Lily Potter (GF34, GF36). More "echoes" of victims would have appeared if the connection had held, according to Dumbledore (GF36).
- Harry's holly wand absorbed power from Voldemort's yew wand in the graveyard, and even though the Dark Lord switched to Lucius Malfoy's wand during the Battle of the Seven Potters, the Phoenix core still recognized him as a major threat and made the other wand explode (DH1, DH4, DH35).
- When twin wand cores are used in tandem against a common enemy, their power "increases tenfold" (Pm).
- Chadwick and Webster Boot, the adopted sons of Isolt Sayre, founder of Ilvermorny School, each had wands with River Serpent Horn cores. When the evil witch Gormlaith Gaunt attacked the school, the Boot brothers were able to hold her off for a long time due to the combined magical force of the twin wand cores (Pm).
The main reason Voldemort took Lucius Malfoy's wand and kept searching for the Elder Wand was that he feared facing Harry's wand with the twin wand core again after the Reverse Spell Effect in the graveyard. He probably would not have dueled with Harry in the graveyard if he had known the effect was going to be so powerful and make him look weak in front of the Death Eaters. And no one, not even Ollivander or Dumbledore, expected Harry's holly wand to "remember" Voldemort and blow up Lucius Malfoy's wand during the Battle of the Seven Potters. As Dumbledore explained in during the vision at King's Cross: "...you and Lord Voldemort have journeyed together into realms of magic hitherto unknown and unprecedented, and no wandmaker could, I think, ever have predicted it or explained it to Voldemort" (DH35).