The structure behind the twelve signs, what each layer of a chart is actually doing, and how far a sun sign reading can reasonably go.
Elements and modalities
The twelve signs divide into four elements, fire, earth, air and water, describing temperament, and three modalities, cardinal, fixed and mutable, describing how each sign handles change.
Cardinal signs start things, fixed signs sustain them and mutable signs adapt. Element and modality together explain most of what a sign is like before you look at any individual chart, and no two signs share the same combination.
Sun, moon and rising
The sun sign describes the core drive and is the one everyone knows. The moon sign describes emotional reactions and what you need in private, and it is often the sign people recognise more once they read it.
The rising sign, or ascendant, depends on your birth time and describes the manner you arrive with. Together the three explain why two people with the same sun sign can feel nothing alike.
Ruling planets
Each sign is governed by a planet that colours its behaviour. Mars gives Aries its directness, Venus gives Taurus and Libra their pull towards comfort and balance, Saturn gives Capricorn its patience with long timelines.
The ruling planet is also where the traditional correspondences come from, which is why the lucky metal, stone and number of a sign are not arbitrary.
Lucky numbers, colours and metals
Every sign carries a set of correspondences inherited from its ruling planet: numbers, a metal, one or more stones, a flower, a colour range and a day of the week. Mars gives Aries iron and the number nine, Venus gives Taurus and Libra copper, the Moon gives Cancer silver and pearl, Saturn gives Capricorn lead and the number eight.
Each sign page lists the full traditional set, and the lucky numbers pages add a daily draw on top of it for people who prefer something that changes.
How compatibility is scored
Compatibility here is built from three measurable things: whether the elements support each other, whether the modalities compete, and the angle between the two signs on the wheel. Signs four apart tend to flow, signs three apart tend to grate productively, and signs opposite each other attract and exhaust in equal measure.
A low score is not a verdict. It describes where two people have to be deliberate, which is often more useful than being told a pairing is easy.
Cusp birth dates
Sign boundaries move by a day or so each year because the solar year is not a whole number of days. Someone born on the twenty first of a month can genuinely belong to either sign depending on the year and the time of day.
There is no such thing as being half of both. The sun was in one sign at the moment you were born, and the calculator resolves it from your date, time and place.
Retrogrades and transits
A retrograde is an optical effect, a planet appearing to move backwards from where we stand. Traditionally it marks a period for revisiting rather than launching, which is why Mercury retrograde has its reputation for admin and communication going sideways.
Transits are simply where the planets are now against where they were at your birth. They give the daily and monthly readings their texture and they are the reason a horoscope changes at all.
How far a sun sign reading goes
A sun sign horoscope addresses one twelfth of the population at a time, so it works best as a prompt rather than a prediction. If a reading points at patience, the useful question is where you are currently forcing something.
For a picture that fits you rather than a twelfth of everyone, add the moon and rising signs. That is usually the point at which people stop finding astrology generic.