House Cusps

The fuller picture

A cusp is a boundary, not a zone. Each of the twelve is a single degree and minute of the zodiac, and the house runs from its own cusp forward to the next one. Four of them are the chart's angles — the first cusp is the ascendant, the tenth is the midheaven, and the seventh and fourth are the points opposite them. The other eight are the intermediate cusps, and they are the ones different house systems argue about.

The most load-bearing thing a cusp does is hand the house a ruler. Whatever sign is on the cusp, the planet that rules that sign becomes the ruler of the house, and that is the step that turns a topic into something you can actually judge: the matter asked about is assigned a house, the house cusp names a sign, and the sign names the planet you then follow through the chart. It works the same way for every house, and it is why the exact degree of a cusp matters — a cusp two degrees earlier can sit in the previous sign and hand the question to a different planet entirely.

A sign merely present in a house does not rule it, and this catches experienced people out. Quadrant house systems produce houses of unequal size, so at higher latitudes a house can be wide enough to swallow a whole sign that never touches either cusp — one worked chart has an eleventh house running from 17° Pisces to 11° Taurus, fifty-four degrees wide, with the whole of Aries inside it. A seventeenth-century astrologer read that chart and called Mars the lord of the eleventh. The correction is that Pisces is on the cusp, so Jupiter rules; Mars, ruling the enclosed sign, is at most a co-ruler, and a co-ruler is weaker and cannot stand as the significator by itself. There's a wider version of the same idea in circulation — that any sign with a presence in the house has some bearing on it, not only a wholly enclosed one — and the two versions are not the same rule. Our own position is stricter than either: rulership comes from the cusp sign alone.

Cusps are also sensitive points in their own right, with two consequences. A planet within five degrees of the next cusp, and in the same sign as that cusp, is read as belonging to the house it is entering rather than the one it is technically in — that rule decides house placement, and house placement can decide a verdict. And a planet sitting on a cusp takes on that house's business, though the contact is a conjunction and nothing more: a cusp has no body, so no aspect runs outward from it, exactly as with the Arabic parts.

On house systems, the practical answer is short. Regiomontanus is the traditional choice for horary and the app's default, and the systems only disagree about the eight intermediate cusps — the ascendant and midheaven are the same in all of them. When we back-solved the printed charts in our sources, the seventeenth-century figures reproduced far better under Regiomontanus than under Placidus, in one case by a factor of about seventy-seven across twelve cusps, while the later English figures reproduced under Placidus instead. That is evidence about those astrologers' practice at those latitudes rather than a proof about the sky.