Two people cast the same chart on two sites and get different house placements. Same planets, same signs, same aspects, but Venus is in the 6th on one and the 7th on the other. Nobody made a mistake. The two sites use different house systems, and the difference is the most common source of confusion for people moving past the beginner stage.
This guide explains what a house system is, how Whole Sign and Placidus each divide the sky, where they agree and disagree, which techniques need which system, and how to choose without pretending one is simply correct.
What a house system decides
The Ascendant and Midheaven are astronomical facts: where the ecliptic crosses the horizon and the meridian at your birth. Every system agrees on them. What the systems disagree on is how to divide the rest of the circle into twelve houses starting from those points.
That division decides which house each planet lands in, and therefore what area of life it describes. It doesn't change signs, aspects, or the angles themselves. So the stakes are real but bounded: house systems change the "where," never the "what."
Whole Sign: one sign, one house
The oldest system in Western astrology, standard in the Hellenistic period and in Indian (Vedic) astrology to this day. The rule is simple: the sign the Ascendant is in becomes the whole 1st house. The next sign is the whole 2nd house. And so on. Every house is exactly 30 degrees and coincides with a sign.
If you have 25 degrees Leo rising, all of Leo (0 to 30) is your 1st house, all of Virgo is your 2nd, and so on. A planet at 2 degrees Leo is in the 1st house even though it rose hours before your Ascendant degree.
The Midheaven in Whole Sign is a point that floats: it lands somewhere in the 9th, 10th, or 11th house depending on latitude and time of year, and it keeps its meaning (career, public standing) as a sensitive point rather than as the cusp of the 10th.
Placidus: houses by time
The default in most Western software since the twentieth century. Developed in the seventeenth century by Placidus de Titis, it divides the circle by time: each house cusp is the degree of the ecliptic that has completed a fixed fraction of its journey from horizon to meridian. The Ascendant is the 1st cusp, the Midheaven is exactly the 10th cusp, and the intermediate cusps are calculated to fall between them in proportion to time.
The result: houses of unequal size. At mid-northern latitudes, houses near the horizon can be over 40 degrees while houses near the meridian are under 25. The cusps fall mid-sign, so a house typically spans parts of two signs, and sometimes contains a whole sign that touches no cusp (an interception).
Placidus is what most people have been looking at without knowing it.
Where they agree
More often than the argument suggests. For most planets in most charts, both systems put them in the same house or an adjacent one. Planets in the middle of a sign, away from any Placidus cusp, typically match. The Ascendant sign is always the 1st house sign in both.
If you compare your chart in both systems and eight of ten planets stay put, that's normal.
Where they disagree
The planets near a Placidus cusp are the ones that move. A planet at 28 degrees of a sign might be in the 6th house in Whole Sign (because the whole sign is the 6th) but the 7th in Placidus (because the 7th cusp fell at 25 degrees of that sign).
The Midheaven is the other big difference. In Placidus it's always the 10th cusp. In Whole Sign it can sit in the 9th or 11th, which changes how you read career. Whole Sign users read the 10th sign for career and the Midheaven as an extra emphasis point; Placidus users read the Midheaven as the career itself.
And the sizes: in Placidus, a big house holds more planets by geometry. Whole Sign distributes evenly.
Intercepted signs and the high-latitude problem
Placidus has a known weakness: at high latitudes (roughly above 60 degrees, so Scandinavia, northern Canada, Alaska), the time-based math breaks down and some cusps can't be calculated. Software substitutes another system or produces distorted houses. Whole Sign has no such limit; it works at any latitude.
Even at moderate latitudes, Placidus produces intercepted signs: a sign entirely inside a house with no cusp, and its opposite sign likewise. Some readers treat interceptions as meaningful (energy that's "locked in"); others treat them as a geometric artefact. Whole Sign has no interceptions by construction.
Which techniques need which system
This is the practical answer, and it's less about belief than about what the technique was built on.
Whole Sign is required for: annual profections, zodiacal releasing, and most Hellenistic timing techniques. They were designed with whole signs and produce ambiguous results otherwise. Astrolas runs profections and zodiacal releasing in Whole Sign regardless of your natal setting.
Placidus is the convention for: most modern natal delineation, most twentieth-century textbooks, and most psychological astrology. If you're learning from modern Western sources, their house descriptions assume Placidus or something close to it.
Either works for: transits to planets, aspects, synastry by aspect, progressions by aspect. Anything that doesn't depend on house placement is system-independent.
Many practitioners now use both: Whole Sign for the traditional timing techniques and as a first read, Placidus (or Koch, or Porphyry) for the natal detail. That's not indecision; it's using each tool where it was designed to be used.
How to choose
Three honest options:
- Use Whole Sign if you're drawn to traditional astrology, if you use profections or releasing, if you were born at high latitude, or if you like the simplicity of one sign per house. This is the fastest-growing choice among practitioners.
- Use Placidus if you're learning from modern books, if you want the Midheaven as the 10th cusp, or if your chart's Placidus placements match your life better than Whole Sign's.
- Test both on your own chart against known life events. Take a planet that moves house between systems and ask which house description fits your actual experience of that planet. Most people find one system "reads" better for them, and that's a legitimate way to decide.
What not to do: assume the system your first app used is the only one, and argue with people who got a different house from a different system.
Compare them on your chart
Open the birth chart tool โ. The wheel has a house system selector; switch between Placidus and Whole Sign and watch which planets change house. The angles and aspects stay fixed.
Make a note of every planet that moves. Those are the only placements where the choice matters for you. If none move, you can stop worrying about house systems entirely. If two or three move, read both house descriptions for each and decide which describes your life. That's the whole decision, and it's yours to make rather than a software default's.