A geometric description is given for the Sp(2) covariant version of the
field-antifield quantization of general constrained systems in the Lagrangian
formalism. We develop differential geometry on manifolds in which a basic set
of coordinates (`fields') have two superpartners (`antifields'). The
quantization on such a triplectic manifold requires introducing several
specific differential-geometric objects, whose properties we study. These
objects are then used to impose a set of generalized master-equations that
ensure gauge-independence of the path integral. The theory thus quantized is
shown to extend to a level-1 theory formulated on a manifold that includes
antifields to the Lagrange multipliers. We also observe intriguing relations
between triplectic and ordinary symplectic geometry.Comment: Revised version -- our treatment in Section 5 has been extended and
several pedagogical notes inserted in Sections 2--4; more references added