The OMU Trigger:

The OMU is also used in triggering. An on/off signal is derived for each of the 8 ``towers'' (the three views in each region) by requiring a hit in two of the three views. The OM₁ signal requires one tower to be on. The OM₂ signal requires two non-adjacent towers to be on. (Because of the overlap, a single muon could trigger two adjacent towers.)