Magical Quill

"It detects the birth of a magical child and writes his or her name down in a large book. Every year, Professor McGonagall checks the book and sends owls to the children who are turning eleven, to inform them that they have a place at Hogwarts."

- Description from Pottermore.com

The Magical Quill was a quill at Hogwarts School of Witchcraft and Wizardry that detected the births of all magical children and wrote their names down in a big, parchment book.

Use
Since this record contained the name and birthdate of every magical child, it was used to determine who was eligible to attend Hogwarts, and when. Every year, at least during her tenure as Transfiguration teacher and Deputy Headmistress, Minerva McGonagall would check the book and send an acceptance letter by owl to children who were approaching their eleventh birthdays, letting them know of their place at the school.

Behind the scenes

 * It is assumed (though not explained) that this quill only records magical births within the United Kingdom.
 * It is unknown why Professor McGonagall is the one to check the book, possibly due to her status as Deputy Head (and later Headmistress).
 * It is not known if this tradition of tracking magical births with the Quill existed prior to McGonagall's career at Hogwarts, whether it was her idea/replacement for an older system or even if it continues to be used after her retirement.
 * On Pottermore, the Magical Quill was used as a way to register people on the website early, from July 31st-August 6th, 2011. People had to to solve a riddle on the homepage and add the answer to the riddle to the end of the special, otherwise inaccesable URL http://quill.pottermore.com/ to obtain a chance at early registration for the site. If they answered the riddle correctly, theMagicquill.png they were normally redirected to a Harry Potter related website to search for the Magical Quill. If they found it, they had to do something to it, such as click on it or levitate it. If they were successful they were allowed to register for the website prior to the scheduled public launch on October 1st, 2011. There were seven riddles, one each day for the duration of the week. Each one related to one of the different Harry Potter books. Day one's riddle was about Philosopher's Stone, day two's was about Chamber of Secrets, and so on. The riddles became progressively easier as the week goes along, with the riddles for the first three days being more difficult than those for the last four days.

Appearances

 * J.K.Rowling Official Site
 * Pottermore

Notes and references
Plume Magique