Abstract It is apparently well known that one particularly delicate tolerance in designing a tomographic machine is that on the error in alignment of the graticule. If the graticule, or strip which determines the location of the parallel measurements in each view, is displaced by as little as 0.05 mm consistently in each view, a characteristic artifact resembling a tuning fork can appear in the neighborhood of a small dense object (e.g., the petrous bone). In this paper, we give a mathematical analysis of this artifact, leading to a simple quantitative estimate of this error in terms of the displacement. We also show that a rough correction can easily be made in software to remove this artifact by making the opposite shift in the weight function used. We show further that the displacement can be indirectly measured by using a pin phantom. Finally, we note that if the displacement were measured directly using, e.g., a triangular piece of lead, the above correction would then remove the artifact and eliminate the need for such a delicate tolerance.