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 Mercury in your third house you're a woman or man of ideas, which you can express with great talent. You have a fine mind (if Mercury is not strongly afflicted), and are curious about everything. Teaching, research, or investigative work is appropriate with Mercury in your third house. You enjoy studying and communication in all its forms: spoken, written, via computers, etc., and are good with detail. There's probably a lot of movement and travel, albeit not necessarily over long distances, in your life.

Venus in Mercury's natural house (the third) lends harmony, balance and an aesthetic style to the mind. Artistic and creative, you're a pleasant person who loves art and music. All things being equal—which they rarely are—your early life was probably a comfortable one, and you had good relationships growing up, especially with siblings (if any). Often mentally lazy, you still love to communicate and pursue conversations, but you hate to argue.

If Mercury or Venus 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 either planet 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 Mercury or Venus 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, that planet expresses itself primarily—and eventually entirely—in the fourth house.