The Secret Annex was the hidden living space where Anne Frank, her family, and another Jewish family resided in hiding during the Nazi occupation in Amsterdam. It was located in the upper floors of the building where Anne's father, Otto Frank, had his business. It was concealed by a sliding bookcase and had very limited space. Anne began writing her famous diary, expressing her thoughts, feelings, and experiences, in this secret annex. This diary, known as "The Diary of a Young Girl" or "Anne Frank: The Diary of a Young Girl," provides a firsthand account of the life of a Jewish girl during the Holocaust and has become an important piece of literature that sheds light on the human experience during that difficult period.