  Light carries intrinsic spin angular momentum (SAM) when the electric or
magnetic field vector rotates over time. A familiar vector equation calculates
the direction of light's SAM density using the right hand rule with reference
to the electric and magnetic polarisation ellipses. Using Maxwell's equations,
this vector equation can be decomposed into a sum of two distinct terms, akin
to the well-known Poynting vector decomposition into orbital and spin currents.
We present the first general study of this spin decomposition, showing that the
two terms, which we call canonical and Poynting spin, are chiral analogies to
the canonical and spin momenta of light in its interaction with matter. Both
canonical and Poynting spin incorporate spatial variation of the electric and
magnetic fields and are influenced by optical orbital angular momentum (OAM).
The decomposition allows us to show that the OAM of a linearly polarised vortex
beam can impart a first-order preferential force to chiral matter in the
absence of spin.
