Check in - check out board. When a miner enters the mine he places his brass tag on the "in-board." When he leaves the mine, he moves the tag to the "out-board." The brass tag bears the miner's social security number, which matches a second tag rivited to his belt. This is used for position identification in case of a catastrophe. This system allows people on the surface to know who is underground at any given time, likewise, who is off shift.

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