  In the (2+2) formulation of general relativity spacetime is foliated by a
two-parameter family of spacelike 2-surfaces (instead of the more usual
one-parameter family of spacelike 3-surfaces). In a partially gauge-fixed
setting (double-null gauge), I write down the symplectic structure of general
relativity in terms of intrinsic and extrinsic quantities associated with these
2-surfaces. This leads to an identification of the reduced phase space degrees
of freedom. In particular, I show that the two physical degrees of freedom of
general relativity are naturally encoded in a quantity closely related to the
twist of the pair of null normals to the 2-surfaces. By considering the
characteristic initial-value problem I establish a canonical transformation
between these and the more usually quoted conformal 2-metric (or shear) degrees
of freedom. (This paper is based on a talk given at the Fifth Midwest
Relativity Conference, Milwaukee, USA.)
