The three-dimensional power spectrum of a helical filament is shown (A). The power spectrum from a cryo-EM image of a helical filament (where the image is a projection of a three-dimensional object onto a two-dimensional plane) is the central section of the three-dimensional power spectrum, and this would be given by the intersection of the red plane in (B) with the power spectrum. The yellow arrows in (B) show the intersection of this plane with a layer plane that arises from a 1-start helix. The intensities on this central section are shown in (D), which would be the power spectrum from untilted filaments, i.e., those whose filament axis is parallel to the plane of projection. The yellow arrow in (D) shows the layer line from the 1-start helix, with peak intensities on both sides of the meridian. Now consider what happens when the filament axis is tilted away from parallel to the plane of projection. The central section (C) would now intersect the 1-start layer plane at the positions given by the two yellow arrows, and in the two-dimensional power spectrum the two peaks would collapse to a single peak on the meridian (E, yellow arrow). If one did not know that the filament was tilted, such intensity would be mistaken for a meridional layer line with the Bessel order n = 0.