Overlay
every reader
What the tradition reads
An overlay places one person's planets into the house wheel of another person's chart, asking not what sign or degree a partner's planet falls at but which of the other person's twelve houses it lands in once the two charts share a single set of house cusps. Read this way, a partner's Venus falling in one's own fifth house is treated as that partner activating romance and creative pleasure in the native's life specifically, quite apart from anything a straightforward comparison of the two charts' aspects to each other would show, and the technique is generally run in both directions at once, each partner's planets read against the other's houses in turn, since the two overlays are rarely symmetrical. The tradition treats the overlay as a distinct layer of synastry alongside the direct aspects between the two charts: where an aspect between two planets shows a fairly straightforward exchange, each side playing a recognisable part, the effect on the person whose house is activated by an overlay is read as less straightforward and more one sided, useful in particular for locating which specific department of a life, work, home, children, one partner is felt to bring alive in the other.
The story
Overlay carries its plain English sense, to lay one thing over another so that the lower layer still shows through, an image that describes exactly what the technique does: one chart's planetary positions are laid over the fixed grid of the other chart's houses, so the shape of the houses does not change but what falls inside each one does. The underlying idea, reading a second chart's planets by which houses of a first chart they occupy, is a natural and probably very old extension of ordinary horoscopic house theory, since any astrologer comparing two charts side by side could in principle ask this question the moment houses themselves were in use. Cafe Astrology, whose own page supplies the technique's worked examples, introduces it simply as an important technique that astrologers use in synastry, without dating it or naming an originator, and Wikipedia's general article on astrological compatibility describes synastry broadly, as the branch of astrology that shows the compatibility of romantic partners, without using the word overlay at all. This corpus has no reachable source that documents when the term overlay was coined or by whom, and states that plainly rather than guessing at a history neither page gives.
Written from
Corpus key glossary:overlay.