When fung shui masters assess directions, they use units of 15 degrees known as the 24 mountains, which are inscribed on Chinese compasses, or luo pans. Each of the four cardinal (for example, south) and four intercardinal directions (such as northwest) are divided into three units to obtain the 24 mountains. The 12 zodiac animals account for half of these mountains while the remaining 12 are represented by eight yin and yang element markers and four trigrams of the I Ching. The markers include those of water, wood, fire and metal but exclude earth, which symbolises the centre, so is not ascribed to any direction. The four I Ching trigrams indicate the centre of the intercardinal sectors, that is, southwest, southeast, northwest and northeast. Four of the 12 animals indicate the centre of the cardinal directions. For instance, the rat represents north (352.5 to 7.5 degrees) while the horse represents south (172.5 to 187.5 degrees). The other eight animals are assigned to the intercardinal directions. For example, the ox represents 22.5 to 37.5 degrees and the tiger 52.5 to 67.5 degrees. The animal sectors are used to identify directions that should be avoided. For instance, a person should not sleep with their head pointing to or sit with their back to the direction that clashes with their year of birth.