Partly because of this deliberate design decision to help shield users from component failures, the few shared markers contain a great deal of self-checking circuitry. This is feasible because there are only a few markers, and beneficial because their correct function was critical. Digit codes, for instance, are checked to ensure that exactly two out of the five lines are activated. When the built-in self-test circuits of a marker detect an error, a large punch card is produced at the test station recording the failure in order to assist the switchmen in detecting it and diagnosing the source. The card punch, with some cards, is visible in the lower left of the test frame picture below.