The word 'aspect' comes from the Latin aspicio, 'to regard'.
Aspects are measured in celestial longitude and aspect meanings are affected by
the geometrical and numerical relationships they make within the zodiacal
circle: hence, what is usually called the square aspect today was traditionally
called the quadrate or quadrangular aspect (referring to the plane figure that
the aspect is able to create) or the tetragonal aspect, referring to the number
of sides that its associated geometrical figure possesses (from the Greek
The traditionally recognised aspects include: conjunction (0˚), sextile
(60˚), square (90˚), trine (120˚) and opposition (180˚). The easiest way to recognise aspectual relationships is to consider how signs related by a square are all part of the same quadruplicity, and signs related by trine are all part of the same triplicity (as shown in this PDF file).