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The number of class averages (one class average contains images that are the same) resulting from the multivariate statistical analysis of the individual images of particles are 2d projections of a 3d structure in different projection directions. Given a sufficiently large number of good 2d projections the 3d structure can be reconstructed knowing the orientational relationship between all the projection class averages. For an entirely asymmetric particle at least three different projections are required to solve the orientation problem.