What is Placidus House System?
Developed in the 17th century, the Placidus system divides houses based on the time it takes for each degree of the ecliptic to rise from the horizon to the midheaven. This creates unequal house sizes that vary with latitude. Placidus works well for most locations, but at some extreme latitudes the cusp frame can become unavailable or unusable because some degrees do not rise normally. TriAstra keeps Placidus as the default and uses Porphyry only when the Placidus cusps cannot be used for the specific birth location and time.