What is Ruled join?

In algebraic geometry, given irreducible subvarieties V, W of a projective space P, the ruled join of V and W is the union of all lines from V to W in P, where V, W are embedded into P so that the last n + 1 coordinates on V vanish. It is denoted by J. For example, if V and W are linear subspaces, then their join is the linear span of them, the smallest linear subcontaining them.

The join of several subvarieties is defined in a similar way.