The square is part of the Hellenistic aspect doctrine codified by Ptolemy, its geometry that of the fourfold division of the circle, and its glyph is simply a plain open square, the regular quadrilateral that a ninety-degree division inscribes. Like the trine, the square's sign was not given a dedicated codepoint of its own when aspect symbols were added to the Unicode Standard; it is written instead with the general geometric character WHITE SQUARE at U+25A1, a mark used across mathematics and typography for the same shape, so the aspect glyph and the ordinary drawn square are, in digital text, one and the same character rather than two related but distinct signs, as the conjunction and opposition are.
The mark itself is old and stable: square aspect signs are attested with the rest of the aspect set in the medieval Byzantine codices copying Hellenistic Greek horoscopes, and the glyph needed almost no adjustment when astronomical and astrological tables began to be set in type during the Renaissance, since a square drawn with a straightedge and one set in a printer's font are naturally close to identical. That continuity is part of why the square, alongside the trine, never needed a purpose-built astrological codepoint when the Unicode Standard incorporated aspect symbols: the ordinary geometric shape already did the job a specialised mark serves for the conjunction, opposition, sextile and several of the minor aspects.