Horary astrology is the use of astrology essentially as an oracle to answer a yes-or-no question. The astrologer erects a horoscope for the precise moment and location where the question is asked, and then interprets the horoscope according to a specific set of rules to determine the question's answer.

The person asking the question is referred to as the querent. The matter asked about is referred to as the quesited. In the horary chart, both the querent and the quesited are represented by specific houses of the horoscope; and, through the planetary rulers of those houses, by specific planets as well. Additionally, other people and situations surrounding the question are represented by various houses and planets.

Thus, the horary chart becomes a symbolic diagram of the relationships among the querent, the quesited, and the overall environment and situation. The astrologer can then determine which factors in the environment are working toward a positive or negative resolution of the matter; and, by weighing those factors according to the horary rules, reach a yes-or-no conclusion in answer to the question.

It is generally considered that a question must be of specific pressing importance to the querent in order to be a valid subject for a horary reading. A querent who is about to undergo a serious surgical procedure and asks, "Will this surgery be successful?" is likely asking a question of pressing and immediate importance. By contrast, a casual "Will I be rich someday?", if the astrologer pursues the question at all, is likely to yield an "invalid" chart -- i.e., a chart which, according to the rules of horary, cannot be read for a yes/no answer. Invalid charts in horary tend to occur in situations where the querent is not sincere in asking the question, where the question is casual or irrelevant, or where something is about to happen (or has already happened) that will cause the question to become moot.

Indeed, the first step in any horary reading is to determine whether or not the chart is valid to be read at all. Again, there are specific rules for determining a chart's validity, and if the chart is determined to be invalid, the reading stops right there. The chart itself provides the reason why it is invalid, of which the most common are: The question is premature, in that something is about to happen which will change it or render it moot. The question is too late, in that the situation is already too far along for the question to matter (or, the querent already knows the answer). Or the question is irrelevant or frivolous and nothing will come of the matter.

If the chart is determined to be valid, then the astrologer proceeds with the interpretation -- first determining which houses and planets represent which actors in the "drama" surrounding the question; and then proceeding to evaluate their relationships among one another to determine the question's answer.