There are two staircases opening off the common room into the Tower itself: the boys' staircase, on which the seven boys' dormitories are located (one for each year) and the girls' staircase, on which the seven girls' dormitories are located (one for each year). The girls' staircase is under a spell so that if a boy sets foot on it and persists for a few steps, a wailing klaxon goes off and the staircase temporarily Transfigures itself into a stone slide, sending him and anyone else on the stairs sailing to the bottom. According to Hogwarts, A History, the founders believed that girls were more trustworthy than boys, so that while the girls' staircase is enchanted to prevent boys from getting in, the boys' staircase does not carry a reciprocal enchantment. Each dormitory is a round room with windows looking out onto the grounds, and containing four-poster beds.