System characteristics:
placidus: Most popular in modern Western astrology. Divides diurnal/nocturnal arcs into thirds. Fails at latitudes above ~66° (Arctic/Antarctic circles).
koch: Similar to Placidus but uses the Ascendant's pole. Popular in Europe. Also fails at extreme latitudes.
equal: Simplest system. Each house is exactly 30° from the previous, starting from the Ascendant. MC floats independently. Works at all latitudes.
whole-sign: Ancient system from Hellenistic astrology. Each house corresponds to one zodiac sign. House 1 starts at 0° of the sign containing the Ascendant. Works everywhere.
porphyry: Simple trisection of quadrants between angles. Works at all latitudes.
regiomontanus: Divides the celestial equator into 30° segments, then projects onto the ecliptic. Works at most latitudes.
campanus: Divides the prime vertical into 30° arcs. Complex spherical geometry. Works at most latitudes.
Available house systems
Different methods for dividing the celestial sphere into twelve houses. Each system has different geometric rationale and works best in different contexts.