What is a house cusp?
A house cusp is the boundary between two houses. In whole-sign houses, the cusp of each house is the start of its sign, at 0 degrees.
What it is
In other house systems the cusps are at computed degrees, such as the Lagna degree in the equal house system. The site uses whole-sign houses, so its cusps are sign boundaries.
How it is worked out or read
A planet is placed in the house whose cusp is at or before its position. With whole signs, that is the sign counted from the Lagna sign.
Where it appears on this site
The houses and their signs appear in the birth chart. See whole-sign houses.
Cusps in other systems
In most house systems other than whole-sign, each cusp is a computed degree rather than the start of a sign. The Placidus system, widely used in Western astrology, cuts the houses by time, so that houses in high latitudes can be very unequal. The Koch system is similar. Readers who meet these systems should know that the cusp degrees differ from the sign boundaries, and that a planet can change house when the system changes, even without changing sign.
Reading a cusp correctly
A common mistake is to read a cusp as a boundary the planet must pass before it changes house. In whole-sign houses the cusp is simply the start of the sign, so the boundary is clear. In systems with computed cusps, the cusp is a degree within a sign, and a planet just before it belongs to the previous house. Keep track of which kind of cusp you are reading before you count.
Why this site uses sign cusps
This site uses whole-sign houses, so every cusp is at 0 degrees of a sign. The result is that a reader can verify each house by sign alone, with a pencil and the table of signs, and does not need a second calculation for the cusps. The method page explains this choice, and the house article explains how to count from the Lagna sign.
Cusps and the chart drawing
In a drawn chart the cusps are the lines that separate the houses. In whole-sign houses those lines fall exactly on the sign boundaries, which makes the drawing simple to check, while other systems draw cusps at computed degrees, which can look less regular.
Cusps in a learner's drawing
A learner who draws a chart by hand should mark the cusps clearly, since they show where each house begins. Clear cusps make it easy to see at a glance which planets fall in which house, and they reduce the chance of counting errors.
Why the cusp question matters for learners
Many learners first meet house cusps when two calculators disagree about a planet's house. Knowing that the disagreement usually comes from the house system, not from an error, saves a great deal of confusion. A learner who checks which system each source uses can explain the difference in a sentence and move on to the reading.