According to the late Nicholas DeVore, a famous astrologer who wrote the Encyclopedia of Astrology, while “Hayz” in Arabic means just “Space,” astrologically it takes on a special meaning for Horary charts.
In Horary Questions, a masculine diurnal planet above the Earth in a Day Figure, and a feminine nocturnal planet under the Earth in a Night Figure is a Dignity of 1�, and is reckoned fortunate. The Arabians did not conceive it a perfect Hayz except when the masculine planet was in a male Sign, or the feminine planet in a female Sign. Instead, a masculine planet in a male sign had to also be under the Earth by day as its only light, and thus the person denoted by it to be in a state of contentment.Nicholas DeVore, Encyclopedia of Astrology
Under the Earth
I have italicized the “under the Earth by day” because looking at Helm’s Chart, it is not physically under the Earth, it horoscopically in the North, but that is not the same idea as “under earth” because on the chart it changes with the ascendant. So what is under Earth? Houses one through six are always considered to be under the earth whether they are in the North or South of the chart at hand. Marc Jones disagreed with this and went with the physical placement of the planets thus in his system Sun in Gemini in the eleventh house is not under the Earth. but apparent.
Another point in DeVore’s notation is that this only works in “Horary” charts. C. C. Zain agreed there, I do not. Jones did not mention it at all, of course.