The pentagram was considered by the ancient Greeks and medieval astrologers to have supernatural properties and to be a protective symbol. Often in the past people would mark their doors with pentagrams to ward off evil witchcraft and spirits.
In good magic today (with the top pointed upwards) it is often used as a symbol to support transmission and communication with the spirit world. Its five points are often seen to represent the four classic elements of earth, water, air and fire joining with the fifth element of spirit to create life. A five-pointed star (pentagram) with a circle around it is referred to as a pentacle. This circle represents sacred space in which the fifth element of spirit controls the four earthly elements.
Also in the legend of Sir Gawain and the Green Knight a pentacle adorns his shield representing the five knightly virtues of generosity, courtesy, chastity, brotherly love and piety.