From anywhere there are leads and trailers: Zig means Face Right, and Zag means Face Left. When given in a pair, as in Tag the Line Zig Zag, the call applies to a group of two dancers; each leader does the first (Zig in this case), and each trailer does the second (Zag in this case).
If only one is given, it is directed to the leaders, and the trailers do nothing. In 3/4 Tag the Line, Zig, only the outsides would Face Right:
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