Symbol of the Hallows
The symbol of the Deathly Hallows is an image uniting the three Deathly Hallows (the Elder Wand, the Resurrection Stone, and the Invisibility Cloak). The symbol consists of three geometric shapes: a circle circumscribed inside a triangle, with a straight line drawn from the top point of the triangle to the triangle's base. The line represents the Elder Wand; the circle, the Resurrection Stone; and the triangle, the Invisibility Cloak (DH21).
The symbol is found in several places. It is drawn above the title of The Tale of the Three Brothers (DH16), in Hermione's copy of the book (inherited from Albus Dumbledore, who presumably inked it in). It appears in the book's illustration of the Fountain of Fair Fortune as well (BB2). The symbol is also engraved on the Resurrection Stone (DH34), as well as on Ignotus Peverell's grave—the Peverells are the three brothers referenced in Beedle's tale (DH21, DH35); Marvolo Gaunt held that the symbol was the Peverell coat of arms (HBP10). The symbol is also the mark of a Hallows Quester, who wears it to find fellow Questers and hopefully gain help (DH21).
Several people have used the symbol for varying purposes. Grindelwald used the symbol of the Hallows as his mark, and carved it into a wall at Durmstrang when he was a student there; some later Durmstrang students adopted it as a symbol, as well, until confronted by those whose families had been affected by the Dark wizard (DH8). Albus Dumbledore used it to replace the "A" of his name in the letters he wrote to Grindelwald as a teenager (DH20). Xenophilius Lovegood wore the symbol at Bill and Fleur Weasley's wedding, indicating his status as a Quester and hoping to attract any fellow Questers; however, his doing so incensed Viktor Krum, who saw it as Grindelwald's mark (DH8).