Wand core

The core of a wand is a magical substance placed within the length of wood. The materials used for wand cores can vary widely, though certain wand-makers may prefer to use certain materials; for example, Ollivander only uses phoenix feathers, dragon heartstrings, and unicorn tail hairs.

If two wands have cores from the same source — such as the wands of Harry Potter and Lord Voldemort, which both had cores comprised of a feather from Fawkes, Albus Dumbledore's phoenix familiar — they are brothers. This can create the effect of Priori Incantatem if the wands come into conflict with one another.