Among the many tools that mine owners had to keep miners under their thumbs (not counting local police, union-busting goons, and Pinkerton men -- see here), there was the company store. Workers were paid in vouchers, not dollars, redeemable only at the store owned by -- surprise! -- the mine owners themselves. Not only were the store prices usually inflated, but the scrip was useless anywhere else, which meant miners couldn't accumulate savings and move in search of better work.
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Labor Day is brought to you by the people who brought you the forty-hour week, child labor laws, retirement funds, and medical insurance.