What it means
Jyotish reads a marriage in two layers. The birth chart (D1) shows the circumstances. The Navamsa (D9) shows the deeper quality, how the bond holds up over the years. A planet that is strong in both is the most trusted kind of strong.
The rule takes the lord of your 7th house, the planet that speaks for partnership, and asks three things. Is it comfortable in D1: an own, exalted or friendly sign. Is it comfortable in D9. And does it stay out of the 6th, 8th and 12th houses of D1, where tradition links it to conflict and loss.
About 1 chart in 5 passes, and it is read as a partnership with a stable base. A fail is not a verdict. Tradition reads the 7th house together with Venus, Jupiter and the Upapada. Another rule in this set looks at the Upapada.
This rule joins two divisional charts for one planet. It shows how the builder can cross-check D1 and D9 in a single rule.