The exact point where a house begins.
A planet close to a cusp is considered stronger, because it sits on the threshold between two areas of life. Cusps move considerably when the house system changes, which is one reason two programs can give you different houses from the same data.
A planet a few degrees from the threshold
Each house runs from its own cusp to the next one. If your seventh house begins at 14° Libra and Venus sits at 16° Libra, Venus is in the seventh, just past the start: the classic case of a planet "on the cusp", which is read with extra emphasis.
The opposite case is more debated: a planet a few degrees before a cusp. Many astrologers already read it in the house that is about to begin, as if the threshold had depth. How wide that margin should be is a convention. Some set it at around five degrees, some narrow it, some do not use it at all. Your chart gives you the exact position; how you read the borderline is an interpretive choice.
The cusps that matter most: the angles
Not every cusp carries the same weight. In quadrant systems such as Placidus, the first-house cusp is the Ascendant and the tenth-house cusp is the Midheaven. Opposite them sit the Descendant, opening the seventh house, and the Imum Coeli, or IC, opening the fourth. These are the four angles of the chart, and a planet close to one of them is traditionally among the most prominent in the chart.
The intermediate cusps, such as those of the second or eleventh house, are the ones most affected by the house system you choose. That is where two programs can disagree by several degrees.
Frequently asked questions
What does it mean to be born on the cusp?
In everyday use it describes people born in the days when the Sun moves from one sign into the next. Strictly speaking, cusps belong to houses, but the idea is the same: a threshold. The Sun is always in one sign only, though, and the day it changes sign shifts slightly from year to year. Finding yours takes a chart calculated from your date, time and place.
How do I find the house cusps in my birth chart?
The house table lists, for each house, the sign and degree where it begins: that is its cusp. On the chart wheel it is the line dividing one house from the next. Traditionally, the sign on the cusp colours the whole house, even when no planet sits inside it.
Why do my house cusps change with a different house system?
Because each system divides the sky by a different geometric rule. The Ascendant and Midheaven are points that do not depend on the system, but the intermediate cusps move, sometimes far enough to put a planet in a different house. The further your birthplace is from the equator, the larger these differences tend to be.
See it in your own chart
Calculating your birth chart is free: wheel, planets, houses, aspects and a first reading. All you need is your date, time and place of birth.