This is the most commonly used house system in modern Western astrology. The paths drawn for each degree of the ecliptic to move from the Imum coeli to the horizon, and from the horizon to the midheaven, are trisected to determine the cusps of houses 2, 3, 11, and 12. The cusps of houses 8, 9, 5 and 6 are opposite these. The Placidus system is sometimes not defined beyond polar circles (latitudes greater than 66°N or 66°S), because certain degrees are "circumpolar" (never touch the horizon), and planets falling in them cannot be assigned to houses without extending the system. This result is a weakness of the Placidean system according to its critics, who often cite the exceptional house proportions in the higher latitudes.