Seventh son of a seventh son

The seventh son of a seventh son is a concept from folklore regarding special powers given to, or held by, such a son. The seventh son must come from an unbroken line with no female siblings born between and, in turn, be born to such a seventh son. The number has a long history of mystical and biblical significance: seven virtues, seven deadly sins, Seven Sleepers, Seven Heavens, etc. In this case, it refers to a man who is the seventh son of a man who is himself a seventh son.
In some beliefs, the special powers are inborn, inherited simply by virtue of his birth order; in others the powers are granted to him by God or the gods because of his birth order.
Regional variations.
Ireland.
The seventh son of a seventh son is gifted as a healer. The seventh son of a seventh son is part of a more general phenomenon known as the "cure" (sometimes also called the "charm").
United States.
According to Edward Augustus Kendall in "Travels through the Northern Parts of the United States, in the year 1807–1808" while visiting the Newgate copper mine and prison, the author met an innkeep who told him that "there was to be found in the surrounding hills, a black stone, of a certain species, through which a seventh son of a seventh son, born in the month of February, with a caul on his head, can discern everything that lies in the depths and interior of the globe." The author speculates that the importance of mining to the community gave rise to this localized belief.
Latin America.
In some Latin American countries, the seventh sons of a seventh son is believed to be cursed to be a werewolf, "lobizón" or "lobisomem" (the Portuguese word for "werewolf"). To prevent this, the newborn should be baptized in seven different churches. Alternately, he may be baptized under the name "Benito", with his eldest brother (the eldest son of their father) as his godfather. It is important to note that the local myth of the "lobizón" is not connected to the custom that began over 100 years ago by which every seventh son (or seventh daughter) born in Argentina to "legitimately married parents of good conduct and moral character" is eligible to become godchild to the president.
Italy.
In Italian legend, "Ciarallo" was a seventh son of a seventh son who had the power to enchant and recall snakes, and who was immune to snake venom. Ciarallo was not only a seventh son, but underwent a special initiation rite called "inciaramazione". Customarily, one would ask Ciarallo's intercession when a snake was discovered in the house. Ciarallo would answer these requests by attracting the snake with a whistle. He would also perform the "inciaramazione" rite on other people to ensure protection from snakes by spreading a special oil on their arm. Children were led to Ciarallo by their mothers to get protection.
Transylvania.
Raymond T. McNally and Radu Florescu describe the Transylvanian folk belief that "the seventh son of a seventh son is doomed to become a vampire."