Reference · Astrology
What is a T-square?
Kevin Blaxton — software builder; astrology student for 40+ years · figures computed by the OrbHeaven engine
Published · Last revised
A T-square is two planets in opposition that both square a third planet, called the apex. It draws a right-angled “T” on the chart, and the tradition reads the apex — squared from both ends of the opposition — as the focus where the figure’s tension is felt and worked out.
What is a T-square?
Geometrically it is three points at 0°, 180° and 90° — two planets opposite each other (180°) and a third square (90°) to both. It is one opposition and two squares, forming a T. The figure shows the shape and names its aspects, from the engine.
The three planets usually share a modality — cardinal, fixed or mutable — because signs in square and opposition relation do, which colours how the tension is expressed.
Which configurations qualify
The opposition and both squares must hold within orb: OrbHeaven counts an opposition within 8° and a square within 7° (one degree more with the Sun or Moon). Two planets square a third without the opposition between them is not a T-square. A worked example: the Sun at 5° Aries opposite the Moon at 5° Libra, with Pluto at 5° Capricorn squaring both — the apex.
If the apex’s two squares are not both in orb, the figure is incomplete — an opposition with a loose square, not a T-square.
What it is read as
Interpretation, labelled as such: the T-square is read as a chart’s engine — the opposition sets up a tension between two poles, and the apex planet is where that tension is forced into action. The tradition treats it as demanding but productive, the “hard” aspects driving effort and achievement, with the apex planet carrying the most pressure.
A short history
The square and opposition are classical Ptolemaic configurations, but treating their combination as a named figure with a focal “apex” planet is part of the modern, twentieth-century systematisation of aspect patterns. The emphasis on the apex as the figure’s outlet is a modern interpretive development.
Common misconceptions
- Is a T-square the same as two squares?
- No. A T-square needs an opposition between two planets that both also square a third. Two squares without that opposition do not make a T-square.
- Which planet is the apex?
- The one that squares both ends of the opposition. The tradition reads it as the focus — where the figure’s tension is felt and discharged.
References & further reading
- [1] Ptolemy, Tetrabiblos (2nd c. CE) — the square and opposition among the classical configurations.
- [2] Tierney, B., Dynamics of Aspect Analysis — the T-square, the apex planet, and configuration analysis.
- [3] OrbHeaven engine (astronomy-engine), aspects module — the 90°/180° angles and orbs; the shape’s opposition and two squares re-derived in chart-shapes.test.ts.
Suggested citation
Blaxton, K. (2026). What is a T-square. OrbHeaven Reference. https://orbheaven.com/reference/the-t-squareRevision history
- First build (concise). Shared chart-shape diagram (one opposition + two squares, engine-classified); worked example (cardinal T-square, Sun–Moon opposition with a Capricorn apex). Qualification by orb and the focal apex explained; links up to Aspects, The Opposition and The Square. No photo, no interactive.
Related glossary entries
About a minute. Birth time optional, and no account until after you have seen it.
More in the reference library
What is an aspect in astrology? The angles, the orbs, and the geometry
An aspect is the angle between two points in a chart. This is the geometry — the six angles astrologers use, what an orb is, and where the set comes from — with an interactive dial that classifies any separation using OrbHeaven’s engine.
What is an opposition?
An opposition is two planets 180° apart, facing each other across the chart. This explains the geometry, how OrbHeaven counts it within an 8° orb, what the tradition reads into it, and where it comes from.
What is a square in astrology?
A square is two planets 90° apart — a quarter of the circle, three signs apart, in signs of the same modality. This explains the geometry, how OrbHeaven counts it within a 7° orb, what the tradition reads into it, and where it comes from.