The third house represents understanding through information, and what we know as a result of interactions within our community. "Community" here means neighbors, siblings, short journeys, letters, news and rumors. This house rules our rational thought and conscious mind, memory, speech, early education and environment, writing and communication, dexterity and taken-for-granted skills, explorations, investigations, inquiry, and research of all kinds. Planets in the third house describe the quality and quantity of our mental and nervous energy.

Geometrically speaking, the third house is that one-twelfth sector of the local sky clockwise from the nadir (see above picture), and is occupied by planets in the two hours after they pass through the lowest point of the local sky.

With Jupiter in your third house you tend to learn from everything you encounter in your environment, all that is at hand. You probably had a happy adolescence in which you moved a lot, but nevertheless enjoyed good relations with your relatives and neighbors. You may receive favorable opportunities or blessings through your siblings, with whom you probably have good relationships. You're generally optimistic and well liked, but tend to be restless, and, if Jupiter is afflicted, can scatter your energy.

If Jupiter is within 10° of the nadir (the fourth house cusp), then it is conjunct the fourth house cusp and much of the foregoing interpretation starts to become weaker. If Jupiter is in the third house AND within 4° or 5° of the nadir, then it is said to be on the "dark side" of the fourth house, and it influence will be felt in the fourth as well as in the third house. The closer a third house Jupiter is to the nadir, the more it will be interpreted as though it were in the fourth house; when less than 3° from the nadir, a third house Jupiter expresses itself primarily—and eventually entirely—in the fourth house.