The trine belongs to the Hellenistic aspect doctrine given its lasting theoretical form by Ptolemy, its geometry that of the threefold division of the circle, and its glyph is an equilateral triangle, the same figure a compass and straightedge produce by stepping off three equal arcs. Unlike the conjunction, opposition and sextile, the trine's sign was never given a dedicated astrological codepoint of its own when the Unicode Standard was built; it is instead typically rendered with the ordinary geometric character for a triangle, WHITE UP-POINTING TRIANGLE at U+25B3, a shape borrowed wholesale from mathematical typesetting rather than purpose built for astrology, much as the square aspect reuses a plain geometric square.
The glyph itself is old: triangular aspect marks appear alongside the other aspect and planetary signs in the medieval Byzantine codices that preserve Hellenistic Greek horoscopes, and the shape was carried through largely unaltered as astronomical tables moved into print during the Renaissance, needing no further stylisation, since a triangle drawn quickly by hand and a triangle set in type differ hardly at all. That plainness sets the trine apart from the conjunction, opposition and sextile, whose marks had to be distinguished from ordinary geometric figures and so were given purpose-built forms and, eventually, purpose-built codepoints, while the trine and the square could simply borrow shapes mathematics had already fixed.