House system
A rule for drawing the twelve house boundaries. The common systems agree on the Ascendant and differ on the cuts between the angles. Placidus and whole sign are the two you will meet most often.
Dividing the sky into twelve houses sounds simple until you try it: the sky is a sphere, the horizon sits tilted against the zodiac, and there are many defensible ways to slice the one by the other. Each way is a house system, and astrologers have produced dozens: Placidus, Koch, Regiomontanus, Porphyry, equal, whole sign.
They fall into two broad families. Quadrant systems pin the wheel to both the Ascendant and the Midheaven, then divide the four quarters between them, each by its own geometry. Sign-based systems make simpler cuts: whole sign gives each house one full sign. Most planets sit in the same house either way; the arguments are about the ones near boundaries.
No experiment settles which system is correct, so the practical advice is to pick one, learn charts through it, and compare when a placement feels off. Aspicio computes Placidus and whole sign, and falls back to whole sign at polar latitudes where Placidus stops working.