To be invited to study at Hogwarts, you must receive your Hogwarts letter. Closed with a red seal, these mysterious pieces of post are usually delivered by owls, and usually know exactly where you are. Harry's letter even knew he would be in the cupboard under the stairs! And when the Dursleys tried to move him, they always followed. These letters are usually in green ink, and usually written by Professor McGonagall. And how are students chosen? Well, an enchanted quill and book, called the Quill of Acceptance and Book of Admittance dwell in Hogwarts castle, and know exactly when a future student is ready to be invited to Hogwarts.