Typically, the trailing dancer places a hand on the back or
    shoulder of the lead dancer.  This helps keep the unit connected
    together.  The Japanese style is for the lead
    dancer to place their arms comfortably at their side and slightly
    behind them, extending their hands palms up and toward the trailing
    dancer behind while the trailing dancer places their hands in the
    leading dancer's palms.  In either case, the idea is to physically
    connect the dancers so that they stay together as a unit.  At a
    minimum, the Trailer should tap the Leader's shoulder to let the
    Leader know that the Tandem unit is ready. 
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