sentences:  It was necessary to hold a plastic graticule (see picture below right, under that of an MDR) over the paper to find out what the presence of each burn mark indicated.If more than eight call failures were detected in less than 8 minutes, then the critical common control units would be forced to change from the side in service (A or B) to the other side, the automatic 8-minute changeover would be suspended and a prompt alarm would be sent out.In TXE2s, a call which terminated within the same exchange went through seven switching stages, whereas a call going out to another exchange went through just three switching stages.