We analyze Young's interference experiment for the case that two correlated, linearly polarized beams are used. It is shown that even when the incident fields are partially coherent, there are always correlation singularities (pairs of lines where the fields are completely uncorrelated) on the observation screen. These correlation singularities evolve in a non-trivial manner into dark lines (phase singularities in the paraxial approximation). The latter in turn each unfold into a triplet of polarization singularities, namely an L-line and two C-lines of opposite handedness. © 2007 Elsevier B.V. All rights reserved.