Cusp
every reader
What the tradition reads
A cusp is the boundary line marking where one sign or one house ends and the next begins, the exact degree at which the zodiac's continuous circle is cut into the discrete twelvefold divisions astrology reads from it. House cusps are cast individually for a birth chart from the time, date and place of birth, and depending on the house system used they can fall anywhere within a sign, while sign cusps are fixed points, always thirty degrees apart, that never move regardless of whose chart is being read. The tradition is divided on how much weight a cusp itself deserves: some astrologers hold that a planet or a Sun standing within a degree or two of a cusp partakes of both signs at once, reading the person as carrying a blend of the two signs' natures, while others hold that only the exact degree matters and that a planet is wholly in whichever sign contains its precise longitude, with nothing of the neighbouring sign bleeding across the line. A third position allows the cusp some narrow width of influence without extending it as far as a full second sign, typically no more than a degree or two either side of the boundary.
The story
The word cusp comes from the Latin cuspis, meaning a point, a spear point or the sharpened tip of something, an image of a sharp dividing edge rather than a wide zone, which sits somewhat at odds with the later popular sense of a cusp sign as a blended border region. English absorbed the word in its general sense of a pointed or transitional edge well before astrology narrowed it to this specific technical use, keeping the Latin sense of a sharp meeting point rather than a soft gradient. The astrological cusp nonetheless carries a popular reputation as a fuzzy overlap between two signs, the everyday idea of being born on the cusp, a looser sense that coexists uneasily with the word's own sharper Latin root. Astrologers who calculate an exact chart from an exact birth time and place have little practical use for this looser sense, since a computed chart places the Sun or any other body at one precise degree in one sign and not, properly speaking, on the boundary between two; the fuzzy, blended cusp belongs to popular usage rather than to the practice of casting an actual chart.
Written from
Corpus key glossary:cusp.