 The holes in the slices represent weaknesses in individual parts of the system and are continually varying in size and position across the slices.The system produces failures when a hole in each slice momentarily aligns, permitting (in Reason's words) "a trajectory of accident opportunity", so that a hazard passes through holes in all of the slices, leading to a failure.Frosch described Reason's model in mathematical terms as a model in percolation theory, which he analyses as a Bethe lattice.