When two quasiparallel identical beams, originating from the same laser, are superposed, the crossing volume becomes the seat of a system of interference fringes. The spatial coherence of the radiation filling this region may be evaluated by observing the interference pattern generated on a far screen. When the beams originate from two identical but different lasers, the fringe system in the crossing region will be washed out by mutual phase fluctuations: the radiation field, however, retains its ability to produce an interference pattern when it goes through a double slit. This remark may be the starting point for a simpler and intuitive approach to the van Cittert–Zernike theorem. Simple laboratory experiments are described which illustrate these results.